xwayland: add wlr_xwayland_set_workareas()

This function allows compositors to set the _NET_WORKAREA property on
the root window. XWayland clients use _NET_WORKAREA to determine how
much of the screen is not covered by panels/docks. The property is used
for example by Qt to determine areas of the screen that popup menus
should not overlap (see QScreen::availableVirtualGeometry).

include/xwayland/xwm.h

@@ -88,6 +88,7 @@ enum atom_name {
 	DND_ACTION_PRIVATE,
 	NET_CLIENT_LIST,
 	NET_CLIENT_LIST_STACKING,
+	NET_WORKAREA,
 	ATOM_LAST // keep last
 };