cleanup/frames: Move MetaFrame to x11

The PR moves the frame.[c|h] files from core to x11 as well as the frame / frame_bounds / has_custom_frame_extents fields from MetaWindow to MetaWindowX11 similar to !3211 (merged). It also introduces 3 new vfuncs so that the X11 specific bits are handled in MetaWindowX11.

There are still few calls to frame.h functions in window.c like meta_window_ensure_frame / meta_window_destroy_frame / meta_frame_clear_cached_borders / meta_frame_get_frame_bounds that would have to be either gated or refactored somehow.

cc @carlosg, some feedback on the approach would be appreciated.