xwayland: add wlr_xwayland_surface_has_window_type()

The infrastructure to read _NET_WM_WINDOW_TYPE already exists in wlroots (it's used for example in wlr_xwayland_or_surface_wants_focus()). But the window type isn't easily accessible to the compositor because the atoms to compare against are private to xwm.c.

labwc has recently gone to a fair amount of effort (including opening a whole new xcb connection) just to get the needed window type atoms: https://github.com/labwc/labwc/commit/a04b394e591163505b66706ff9ebcadb26871345

It seems much cleaner to add the remaining few (3) atoms to wlroots and implement a shared function which can be used by any wlroots compositor.