window: Split out recalc of skip_taskbar / skip_pager

This will be our first attempt for the MetaWindowX11Private
Start splitting the X11-specific stuff of MetaWindow out into a subclass

This is fairly simple and basic for now, with just skip_taskbar /
skip_pager, but eventually a lot of "WM policy" like this, including
move-resize, will be in subclasses for each individual surface.
Start moving X11 implementation to its own subdirectory

This is specifically about managing X11 windows, not necessarily
running as an X11 compositor. By that I mean that this code is
still used for XWayland windows, and event handling is still and
modesetting / monitor management is still in core/.

This is also a fairly conservative move. We don't move anything
like screen.c or bell.c in here, even though those are really
only for X11 clients.
Move more fields to MetaWindowX11
window: Move urgency to a setter
window: Rename wm_hints_urgent to urgent

The flag and setter is now decoupled from the WM_HINTS state.