window: Move meta_window_move_resize_request to window-x11
window: Move _NET_WM_ALLOWED_ACTIONS setting to window-x11 as well
core: Remove unused function
window: Remove some superfluous uses of window->xwindow
window: Add some hooks for when grab ops begin / end on windows
window: Replace manual switch statement for a resizing grab op
window: Simplify resize_popup handling
window: Turn grab_op_begin / grab_op_ended into vfuncs

And move the grab_resize_popup management to the X11 window class,
as this is only used under X11.
Move sync_request_* handling mostly to window-x11