This commit entirely reworks the internal structure of the entire crate,
as well as some of its APIs. This crate only accepts a C pointer to a
Wayland display object, since the target audience of this crate are
libraries without a wayland-client types. Also since seat information is
not presented in such clients most of the time, the clipboard entirely
relies on its seat tracking.
* Multiple seat support and api refractor
* Remove the need for two GlobalManagers
* Remove unnecessary `Arc<Mutex<_>>` and `if let`s with `unwrap()`
* Use cloned display to create eventloop in `new_threaded()`
* Update docs
* Wrap display to event queue
