Replace DnD threshold calculations to allow checking while off the
actor we're receiving events from.  This fixes the problem of attempting
to drag an item from it's edge, and then off the actor entirely, at which
point the threshold checker would stop receiving events until you went
back over the actor, resulting in a very confusing experience.

This uses a timer based check that is started at the same time as previously,
but it uses global.get_pointer at intervals to compare the current mouse
position with the drag start points, regardless of whether you're still
over the actor or not.