Undo: make weight editing undoable

Implement the EditWeight undo command. Since there is common code
(storage of the old weight), this creates a common base class for
RemoveWeight and EditWeight. The model calls directly into the undo
command, which is somewhat unfortunate as it feels like a layering
violation. It's the easy thing to do for now.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/subsurface-qt/DiveListNotifier.h

@@ -88,6 +88,7 @@ signals:
 	void weightsystemsReset(const QVector<dive *> &dives);
 	void weightAdded(dive *d, int pos);
 	void weightRemoved(dive *d, int pos);
+	void weightEdited(dive *d, int pos);
 
 	// Trip edited signal
 	void tripChanged(dive_trip *trip, TripField field);