xcb errors: optional dependency with improved messages

Now message can look like:
[xwayland/xwm.c:991] xcb error: op ChangeProperty (no minor), code Window (no extension), value 6291465

instead of this one when the lib is not available:
[xwayland/xwm.c:999] xcb error: op 18:0, code 3, sequence 103, value 6291465

The value in case of Window is the window id, so we can tell what
function applied on which window which is a good start.
The sequence ought to be able to tell us more precisely which
invocation it was, but we never log it when calling functions
so is useless in practice and no longer logged.

xwayland/meson.build

@@ -15,6 +15,7 @@ lib_wlr_xwayland = static_library(
 		xcb_image,
 		xcb_render,
 		xcb_icccm,
+		xcb_errors,
 		xkbcommon,
 		pixman,
 	],