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:
a04b394e59

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.

v2: naming updates

include/xwayland/xwm.h

@@ -74,6 +74,9 @@ enum atom_name {
 	NET_WM_WINDOW_TYPE_NOTIFICATION,
 	NET_WM_WINDOW_TYPE_SPLASH,
 	NET_WM_WINDOW_TYPE_DESKTOP,
+	NET_WM_WINDOW_TYPE_DOCK,
+	NET_WM_WINDOW_TYPE_TOOLBAR,
+	NET_WM_WINDOW_TYPE_DIALOG,
 	DND_SELECTION,
 	DND_AWARE,
 	DND_STATUS,