Add Xwayland tests
This series introduces a xwayland client library (based on libxcb) to tests out basic functionality we have currently in Xwayland/XWM.
Some of the tests have the asserts commented out as these require fixing in Xwayland. Specifically, to remove them, we'd need to merge !424 (closed) (kiosk-shell, and incorrect zero size hints) and !614 (closed) (movement of the xwayland surface when coming back from fullscreen).
Further more, some of the tests seem to display a cursor issue (missing) when coming back from maximized/fullscreen to the normal, which I haven't so far elucidated, hence the WIP/Draft mode. Most likely there's a overlap here due the previous issues.
All the commented out asserts have a FIXME tag that points to the issue that needs addressing.