xwayland: Read and publish _NET_WM_STRUT_PARTIAL property

This is needed for compositors that want to reserve space for XWayland panels. Such a feature can be useful in a "transitional" setup, where only the X11 window manager and compositor is replaced but other components of an X11 desktop environment are still used.

This change simply reads the X11 property; the compositor is free to ignore it. Thus, compositors that don't want to support such a "transitional" feature are not impacted.

I can eliminate the xcb-ewmh dependency if that's preferred; it would just require duplicating the xcb_ewmh_wm_strut_partial_t struct.

Example compositor usage: https://github.com/labwc/labwc/pull/625