Linux/x11 input handling (#7811)

Implements the basics of keyboard and mouse handling.
Some keys will need special treatment, like Backspace/Delete. In this
PR, all keys are treated as append-only. Leaving this for a follow-up.

I used @gabydd 's branch as a reference (thank you!) as well as
https://github.com/xkbcommon/libxkbcommon/blob/master/doc/quick-guide.md
For future work, I'll also use
https://github.com/xkbcommon/libxkbcommon/blob/master/tools/interactive-x11.c

All commits are separately compileable and reviewable.

Release Notes:
- N/A

---------

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

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

@@ -181,7 +181,7 @@ impl WaylandWindowState {
 }
 
 #[derive(Clone)]
-pub(crate) struct WaylandWindow(pub(crate) Arc<WaylandWindowState>);
+pub(crate) struct WaylandWindow(pub(crate) Rc<WaylandWindowState>);
 
 impl HasWindowHandle for WaylandWindow {
     fn window_handle(&self) -> Result<WindowHandle<'_>, HandleError> {
@@ -212,7 +212,7 @@ impl PlatformWindow for WaylandWindow {
 
     // todo!(linux)
     fn scale_factor(&self) -> f32 {
-        return 1f32;
+        1f32
     }
 
     //todo!(linux)