drag-action: Added x-drag-threshold and y-drag-threshold properties

drag-action: Added x-drag-threshold and y-drag-threshold properties

                *** This is an API change ***

Replaced the original drag-threshold property with two separate
horizontal (x-drag-threshold) and vertical (y-drag-threshold)
thresholds.

It is some times necessary to have different drag thresholds for the
horizontal and vertical axes. For example, when a draggable actor is
inside a horizontal scrolling area, only vertical movement must begin
dragging. That can be achieved by setting the x-drag-threshold to
G_MAXUINT while y-drag-threshold is something usual, say, 20 pixels.

This is different than drag axis, because after the threshold
has been cleared by the pointer, the draggable actor can be dragged
along both axes (if allowed by the drag-axis property).

http://bugzilla.clutter-project.org/show_bug.cgi?id=2291

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>