dragging: instead of using a custom event loop, use libev

dragging: instead of using a custom event loop, use libev

This is done by installing a new check watcher that replaces the main
X11 event handler and calling ev_run with EVRUN_ONCE until the dragging
loop left state DRAGGING.

With this commit, other handlers, most notably the redraw handler for
placeholder windows, get a chance to run when dragging (placeholder!)
windows around.