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>

commands/command.cpp

@@ -6,6 +6,7 @@
 #include "command_edit.h"
 #include "command_edit_trip.h"
 #include "command_event.h"
+#include "command_pictures.h"
 
 namespace Command {
 
@@ -359,4 +360,11 @@ void addGasSwitch(struct dive *d, int dcNr, int seconds, int tank)
 	execute(new AddGasSwitch(d, dcNr, seconds, tank));
 }
 
+// Picture (media) commands
+
+void setPictureOffset(dive *d, const QString &filename, offset_t offset)
+{
+	execute(new SetPictureOffset(d, filename, offset));
+}
+
 } // namespace Command