undo: implement undo of setting a picture time by drag&drop

Even though the functionality is seemingly trivial, this is a bit
invasive, as the code has to be split into two distinct parts:
1) Post undo command
2) React to changes to the divelist

Don't compile that code on mobile.

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

core/subsurface-qt/divelistnotifier.h

@@ -116,6 +116,9 @@ signals:
 	// Event-related signals. Currently, we're very blunt: only one signal for any changes to the events.
 	void eventsChanged(dive *d);
 
+	// Picture (media) related signals
+	void pictureOffsetChanged(dive *d, QString filename, offset_t offset);
+
 	// This signal is emited every time a command is executed.
 	// This is used to hide an old multi-dives-edited warning message.
 	// This is necessary, so that the user can't click on the "undo" button and undo