Make handling of windows that don't respond to _NET_WM_SYNC_REQUEST reliable

Previously, we were handling failure to respond to _NET_WM_SYNC_REQUEST
in the code path for throttling motion events. But this meant that
if a window didn't respond to _NET_WM_SYNC_REQUEST and there were no
motion events - for a keyboard resize, or after the end of the grab
operation - it would end up in a stuck state.

Use a separate per-window timeout to reliably catch the failure to respond
to _NET_WM_SYNC_REQUEST.

https://bugzilla.gnome.org/show_bug.cgi?id=694046