Implement the Power Rule violation icon

Now when there's a violation of the USB PD power rules, an icon appears
to alert the user to their charger's noncompliance.  The icon has a
tooltip explaining its meaning.

data/pd-buddy-gtk.ui

                                                 <property name="visible">True</property>
                                                 <property name="can_focus">False</property>
                                                 <property name="halign">start</property>
-                                                <property name="hexpand">True</property>
                                                 <property name="label" translatable="yes">Source Capabilities</property>
                                                 <property name="justify">right</property>
                                               </object>
                                                 <property name="top_attach">0</property>
                                               </packing>
                                             </child>
+                                            <child>
+                                              <object class="GtkImage">
+                                                <property name="visible">True</property>
+                                                <property name="can_focus">False</property>
+                                                <property name="icon_name">go-next-symbolic</property>
+                                              </object>
+                                              <packing>
+                                                <property name="left_attach">3</property>
+                                                <property name="top_attach">0</property>
+                                              </packing>
+                                            </child>
                                             <child>
                                               <object class="GtkLabel" id="short-source-cap-label">
                                                 <property name="visible">True</property>
                                                 <property name="can_focus">False</property>
+                                                <property name="halign">end</property>
+                                                <property name="hexpand">True</property>
                                                 <property name="label" translatable="yes">label</property>
                                                 <style>
                                                   <class name="dim-label"/>
                                                 </style>
                                               </object>
                                               <packing>
-                                                <property name="left_attach">1</property>
+                                                <property name="left_attach">2</property>
                                                 <property name="top_attach">0</property>
                                               </packing>
                                             </child>
                                             <child>
-                                              <object class="GtkImage">
-                                                <property name="visible">True</property>
+                                              <object class="GtkImage" id="source-cap-warning">
                                                 <property name="can_focus">False</property>
-                                                <property name="icon_name">go-next-symbolic</property>
+                                                <property name="tooltip_text" translatable="yes">Source violates the USB PD Power Rules</property>
+                                                <property name="halign">start</property>
+                                                <property name="icon_name">dialog-warning-symbolic</property>
                                               </object>
                                               <packing>
-                                                <property name="left_attach">2</property>
+                                                <property name="left_attach">1</property>
                                                 <property name="top_attach">0</property>
                                               </packing>
                                             </child>

pd-buddy-gtk.py

         giveback = self.builder.get_object("giveback-toggle")
         pd_frame = self.builder.get_object("power-delivery-frame")
         output = self.builder.get_object("output-switch")
+        cap_warning = self.builder.get_object("source-cap-warning")
         cap_label = self.builder.get_object("short-source-cap-label")
 
         self.serial_port = serport
         else:
             pd_frame.set_visible(True)
 
-            # Update the text in the capability label
-            # TODO: do this repeatedly
+            # TODO: do these next things repeatedly
+            # Get the Source_Capabilities
             with pdbuddy.Sink(self.serial_port) as pdbs:
                 caps = pdbs.get_source_cap()
+
+            # Update the warning icon
+            cap_warning.set_visible(not pdbuddy.follows_power_rules(caps))
+
+            # Update the text in the capability label
             cap_label.set_text('{:g} W'.format(pdbuddy.calculate_pdp(caps)))
 
         # Show the Sink page