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.
3 jobs for xwayland-strut in 2 minutes and 4 seconds (queued for 5 seconds)