Terminal: pass the voltage across a separation only when it was edited

The three code paths that copied the voltage / protocol to the far
side of a function separated terminal -- the conductor properties
dialog, search/replace and the script API -- compared it against the
far wire's own properties instead of against the edit itself. The two
sides differ on that field just as often as not, so the comparison
held no matter what was edited and every change overwrote the far
side's voltage: setting a colour on a 24 V side dragged the 0 V one
along, where master left it alone. Compare the edit -- what the
dialog was filled with against what the user made of it -- so that a
change of the voltage / protocol is what crosses, which is what the
selection panel multi-edit already did. The values are read before
the undo commands are pushed, since QPropertyUndoCommand applies on
push.

An explicitly selected wire is never the far side of anything:
picking one wire on each side of a separation and editing them edits
them both, in full, however the selection happens to be walked. Nor
is a wire that belongs to a potential on any path, so reaching it
once normally and once across a separation gives it the full edit.

sources/conductorproperties.cpp

@@ -832,6 +832,13 @@ void ConductorProperties::applyChanges(const ConductorProperties &before,
 	protocol describes the electrical node itself, while the text, the
 	function, the wire color... describe one side of the separation and
 	stay local to it.
+
+	Both arguments describe one edit -- what was shown to the user, and
+	what the user made of it -- so that only an edit which really
+	changed the voltage / protocol passes it on. Passing the far wire's
+	own properties as @a before instead would make every edit overwrite
+	its voltage: the two sides differ on that field just as often as
+	not, so the comparison would hold no matter what was edited.
 	@param before : the properties as they were shown to the user
 	@param after : the properties after the user's edit
 */