Print xcb errors

There are two possible messages, with the optional dependency it will look like: [xwayland/xwm.c:991] xcb error: op ChangeProperty (no minor), code Window (no extension), value 6291465

Without it: [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.

This message appears naturally when closing urxvt here, so it's easy to test without messing with the code to generate fake errors... We must be calling xcb_change_property on the window after it got closed?

Fixes #637 (closed).