WIP: Refactor Linux platform implementation (#10227)

This puts the Linux platform implementation at a similar code style and
quality to the macOS platform. The largest change is that I collapsed
the `LinuxPlatform` -> `[Backend]` -> `[Backend]State` ->
`[Backend]StateInner` to just `[Backend]` and `[Backend]State`, and in
the process removed most of the `Rc`s and `RefCell`s.

TODO:
- [x] Make sure that this is on-par with the existing implementation
- [x] Review in detail, now that the large changes are done.
- [ ] Update the roadmap

Release Notes:

- N/A

crates/gpui/src/platform/linux/wayland.rs

@@ -1,9 +1,6 @@
-// todo(linux): remove this once the relevant functionality has been implemented
-#![allow(unused_variables)]
-
-pub(crate) use client::*;
-
 mod client;
 mod cursor;
 mod display;
 mod window;
+
+pub(crate) use client::*;