drag-action: Allow using settings for the drag threshold

drag-action: Allow using settings for the drag threshold

ClutterDragAction should be able to use the newly added ClutterSettings
property exposing the system's drag threshold.

Currently, the x-drag-threshold and the y-drag-threshold properties (and
relative accessors) use an unsigned integer for their values; we should
be able to safely expand the range to include -1 as the minimum value,
and use this new value to tell the ClutterDragAction that it should query
the ClutterSettings object for the drag threshold.

The storage of the properties has been changed, albeit in a compatible
way, as GObject installs a uint ↔ int transformation function for GValue
automatically.

The setter for the drag thresholds has been changes to use a signed
integer, but the getter has been updated to always Do The Right Thing™:
it never returns -1 but, instead, will return the valid drag threshold,
either from the value set or from the Settings singleton.

This change is ABI compatible.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2583