gesture: Make threshold-trigger-edge public

When the threshold-trigger-edge property was introduced in
GestureAction, it was late in the cycle and I elected to keep it
private, given the fact that nobody was subclassing GestureAction
outside of Clutter itself.

These days, people are experimenting more with the GestureAction API, so
they will need access to the various knobs that control the class
default behaviour.

https://bugzilla.gnome.org/show_bug.cgi?id=710227