textview: remove drag selection after dnd move action

textview: remove drag selection after dnd move action

If we have a GDK_ACTION_MOVE, we need to delete the selection. However,
previously this only worked when the drop target and drag source were
different applications, as the selection would get messed up along the
way.

Instead, we stash marks for the duration of the operation so that we can
delete the appropriate selection when completing the move.