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.

src/restore_layout.c

@@ -335,7 +335,6 @@ static void configure_notify(xcb_configure_notify_event_t *event) {
     ELOG("Received ConfigureNotify for unknown window 0x%08x\n", event->window);
 }
 
-// TODO: this event loop is not taken care of in the floating event handler
 static void restore_handle_event(int type, xcb_generic_event_t *event) {
     switch (type) {
         case XCB_EXPOSE: