[PATCH RFC] cursor: add wl_cursor_manager

[Simon Ser]

Ah, I forgot to say that an issue with this design is that some
libwayland-cursor would need to block until the compositor sends the required
metadata (ala-wl_display_roundtrip). I'm not sure if this is an issue or not.

If we don't want to block, then we'll need a much more complicated design with
callbacks or signals, and we wouldn't be able to re-use parts of the current
API.

Thoughts?