Improve flicking behavior of QQuickGeoMapGestureArea

This patch replaces the two separate flick velocities and displacements
for x and y with one vector , so that there will be no discrepancy
between the flicking direction and where the map actually flicks

Change-Id: I83164107003a423cb7332da5759d83ff689c1b31
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>