screen: Fetch the COW window up-front

XGetCompositeOverlayWindow is a round trip to the X server. We shouldn't
be doing it on every MapWindow.
window-props: Use the standard property type system for WM_WINDOW_ROLE

There seems to be no reason we can't just use this.
window-props: Use the standard property type system for _NET_WM_WINDOW_TYPE
window-props: Fix assert testing

I have *no* idea how this even compiled in the first place. What the hell?
window-props: Don't load WM_STATE

Since we don't do anything with it.
window-props: Use a flags-based approach for our prop hooks

This allows us to extend the table better with newer flags.
window-props: Fizzle out PropertyNotifies that happen after initial map

Properties like _NET_WM_DESKTOP and _NET_WM_STATE are supposed to be
ignored after the initial map of the window, so ignore any
PropertyNotifies for these.