Filter: move recalculation of filter from FilterModel to TripModel

The way this was accessed via Qt's model semantics was horrible.
This gives arguably more readable code, since we don't have to
shoehorn things through QVariants.

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

core/subsurface-qt/DiveListNotifier.h

@@ -94,6 +94,7 @@ signals:
 
 	// Filter-related signals
 	void numShownChanged();
+	void filterReset();
 
 	// This signal is emited every time a command is executed.
 	// This is used to hide an old multi-dives-edited warning message.