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.rs

@@ -1,12 +1,14 @@
-mod client;
+// todo(linux): remove
+#![allow(unused)]
+
 mod dispatcher;
 mod platform;
 mod text_system;
-mod util;
 mod wayland;
 mod x11;
 
 pub(crate) use dispatcher::*;
 pub(crate) use platform::*;
 pub(crate) use text_system::*;
-// pub(crate) use x11::*;
+pub(crate) use wayland::*;
+pub(crate) use x11::*;