gstwayland: Move reusable parts of the waylandsink into a library

In preparation for the new element GstGtkWaylandSink, move reusable parts out of GstWaylandSink into the already exisiting but very barebone library.

Notable changes include:

• the GstWaylandVideo interface was dropped
• support for wl-shell was dropped
• lots of renaming in order to match established naming patterns
• lots of code modernisations, reducing boilerplate
• members were made private wherever possible

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This was split out from !1515 (merged) in order to land earlier and avoid rebasing issues for other Waylandsink related MRs.

cc @ndufresne @gkiagia