output,xwayland: Add support for _NET_WM_STRUT_PARTIAL

output,xwayland: Add support for _NET_WM_STRUT_PARTIAL

Account for space taken up by XWayland panels (as indicated by the
_NET_WM_STRUT_PARTIAL property) in the usable_area calculation.

This makes it possible to use labwc in a "transitional" setup, where it
replaces the X11 window manager and compositor, but most other parts of
a existing X11 desktop environment can still be used via XWayland.

(Some remaining drawbacks of such a setup would be the lack of desktop
icons, and native Wayland clients not showing up in X11-based taskbars.)