Added option for 12 V

Quite a few PD supplies give 12 V as a fixed PDO, so it makes sense to
give that as an option in the GUI.  Now it is an option.

data/pd-buddy-gtk.ui

                                     <items>
                                       <item id="voltage-five" translatable="yes">5</item>
                                       <item id="voltage-nine" translatable="yes">9</item>
+                                      <item id="voltage-twelve" translatable="yes">12</item>
                                       <item id="voltage-fifteen" translatable="yes">15</item>
                                       <item id="voltage-twenty" translatable="yes">20</item>
                                     </items>

pd-buddy-gtk.py

             voltage.set_active_id('voltage-five')
         elif self.cfg.v == 9000:
             voltage.set_active_id('voltage-nine')
+        elif self.cfg.v == 12000:
+            voltage.set_active_id('voltage-twelve')
         elif self.cfg.v == 15000:
             voltage.set_active_id('voltage-fifteen')
         if self.cfg.v == 20000: