linux: Add keyboard_layout and on_keyboard_layout_change support (#22736)

No issue, as the functionality is currently not being used in Zed. This
is more of a GPUI improvement.

Currently, `keyboard_layout` and `on_keyboard_layout_change` are already
handled on macOS. This PR implements the same for X11 and Wayland.

Linux supports up to 4 keyboard layout groups (e.g., Group 0: English
US, Group 1: Bulgarian, etc). On X11 and Wayland, `event` provides a new
active group, which maps to the `layout_index`. We already store keymap
state from where we can get the current `layout_index`. By comparing
them, we determine if the layout has changed.

X11:
<img
src="https://github.com/user-attachments/assets/b528db77-1ff2-4f17-aac5-7654837edeb9"
alt="x11" width="300px" />

Wayland:
<img
src="https://github.com/user-attachments/assets/2b4e2a30-b0f4-495c-96bb-7bca41365d56"
alt="wayland" width="300px" />

Release Notes:

- N/A

crates/gpui/src/platform/linux/headless/client.rs

@@ -47,6 +47,10 @@ impl LinuxClient for HeadlessClient {
         f(&mut self.0.borrow_mut().common)
     }
 
+    fn keyboard_layout(&self) -> String {
+        "unknown".to_string()
+    }
+
     fn displays(&self) -> Vec<Rc<dyn PlatformDisplay>> {
         vec![]
     }