Show PDP instead of number of PDOs

Assuming the power supply follows the Power Rules, it's a lot more
informative to show its PDP than the number of PDOs it advertises.  This
is the first step towards implementing the new design for showing the
Source_Capabilities.

README.md

 * GTK+ 3.22
 * Python 3.6
 * python-gobject 3.22
-* pd-buddy-python 0.3.1
+* pd-buddy-python 0.4.0
 
 ## Usage
 

pd-buddy-gtk.py

             # TODO: do this repeatedly
             with pdbuddy.Sink(self.serial_port) as pdbs:
                 caps = pdbs.get_source_cap()
-            cap_label.set_text('{} PDOs'.format(len(caps)))
+            cap_label.set_text('{:g} W'.format(pdbuddy.calculate_pdp(caps)))
 
         # Show the Sink page
         hst = self.builder.get_object("header-stack")