Based on a patch from Paul Davis, inject synthetic enter events directly
into the Quartz event stream, instead of trying to synthesize them in GDK.

This seems to magically fix most combo box popup weirdness, I guess
some code is relying on a specfic order of events, or any other state
imposed by the "proper" code path of events coming in the usual way.

The patch also removes _gdk_quartz_events_send_enter_notify_event()
which is now obsolete.

(sortof cherry-pixked from 979e5061
but needed manual editing because GdkQuartzWindow.c was renamed
and apparently earlier patches not picked correctly/completely)