xcb-client-helper: Add a XCB client helper for tests
Another follow-up from !649, which extracts the XCB wrapper library to handle event state changes. Some initial reviews have been done already in !649, and I've tried to keep them here as well: have the connection being made explicit and use it when creating the x11 window; assert if we already have an event handling in progress.
For the time being this only handles map/unmap/create/destroy/property notify events. Also, there's no screenshot support/functionality in this library. Further improvements will be needed to handle other window states: max/fullscreen and screenshoting.
With it, we migrate test-xwayland to it and bump the CI to use xcb-cursor-dev.