window: Move meta_window_configure_notify to window-x11
window: Remove bogus comment

These fields haven't been around for quite some time.
window: Remove another bogus comment

It's not referring to anything!
window: Remove vfunc slots for signals

These aren't used currently, and we're going to be adding
more vfuncs.
window: Remove meta_window_is_modal

It relies on EWMH concepts. Wayland simply has a MODAL_DIALOG type.
Replace wm_state_modal usage with META_WINDOW_TYPE_DIALOG
window: Remove meta_window_set_gravity as "public" API