listbase: Cancel rubberband if not handling drag

If the drag events are claimed by another gesture (e.g. a GtkDragSource
in an item widget), list base still commits a rubberband selection, for
a rubberband which wasn't even visible yet. This is a problem for the
GNOME Files application which needs both rubberbanding and drag-n-drop.

My previous fix[0] was enough for the case where the event sequence is
claimed right before the first GtkDragGesture::drag-update emission,
but it's useless if the event is claimed later (e.g. after the drag
treashold), because a rubberband already exists by that time.

Therefore, the complete solution requres checking whether the event
sequence is no longer being handled by our gesture, and commit the
selection changes only if it is, but otherwise cleanup the rubberband.

This is what GtkFlowBox does already, so let's do the same here.

[0] commit dc4540fa
13 jobs for wip/antoniof/listbase-rubberband-better-fix in 13 minutes and 47 seconds (queued for 2 minutes and 56 seconds)