Make Cursor/Frame support an optional feature

[hesiod]

I'm working on an overlay (using layer shell) that does not require any mouse input or window decoration. For that use case, the cursor support in Smithay's client toolkit is superfluous. I know that wayland-cursor/xcursor is not a huge dependency, but I'd like to optimize my dependency chain a bit and it would be nice if I could trim Smithay's client toolkit down a bit. I've pushed a Proof-of-Concept version of a cursor feature here: https://github.com/hesiod/client-toolkit/tree/optional_cursor I explicitly didn't create a PR since my version is rather rough - I essentially sprinkled #[cfg(feature = "cursor")] all over the place.