Control the Source Capabilities row's sensitivity

If there are no capabilities to show, it doesn't make sense to let the
user try to open the Source Capabilities dialog.  Now we don't, and we
say "None" instead of "0 W".

data/pd-buddy-gtk.ui

@@ -473,7 +473,7 @@
                                               </packing>
                                             </child>
                                             <child>
-                                              <object class="GtkImage">
+                                              <object class="GtkImage" id="source-cap-arrow">
                                                 <property name="visible">True</property>
                                                 <property name="can_focus">False</property>
                                                 <property name="icon_name">go-next-symbolic</property>

pd-buddy-gtk.py

@@ -271,8 +271,10 @@ class Handler:
         giveback = self.builder.get_object("giveback-toggle")
         pd_frame = self.builder.get_object("power-delivery-frame")
         output = self.builder.get_object("output-switch")
+        cap_row = self.builder.get_object("source-cap-row")
         cap_warning = self.builder.get_object("source-cap-warning")
         cap_label = self.builder.get_object("short-source-cap-label")
+        cap_arrow = self.builder.get_object("source-cap-arrow")
 
         self.serial_port = serport
 
@@ -331,7 +333,14 @@ class Handler:
             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)))
+            if caps:
+                cap_label.set_text('{:g} W'.format(pdbuddy.calculate_pdp(caps)))
+            else:
+                cap_label.set_text('None')
+
+            # Make the row insensitive if there are no capabilities
+            cap_row.set_activatable(caps)
+            cap_arrow.set_visible(caps)
 
         # Show the Sink page
         hst = self.builder.get_object("header-stack")