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/interactive.rs

@@ -307,7 +307,6 @@ impl ScrollDelta {
 }
 
 /// A mouse exit event from the platform, generated when the mouse leaves the window.
-/// The position generated should be just outside of the window's bounds.
 #[derive(Clone, Debug, Default)]
 pub struct MouseExitEvent {
     /// The position of the mouse relative to the window.