gpui: Decouple X11 logic from LinuxPlatform (#7598)

Release Notes:

- Separated Linux platform and X11-specific code, so that we can add
Wayland support now.

---------

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

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

@@ -0,0 +1,15 @@
+use std::rc::Rc;
+
+use crate::platform::PlatformWindow;
+use crate::{AnyWindowHandle, DisplayId, PlatformDisplay, WindowOptions};
+
+pub trait Client {
+    fn run(&self, on_finish_launching: Box<dyn FnOnce()>);
+    fn displays(&self) -> Vec<Rc<dyn PlatformDisplay>>;
+    fn display(&self, id: DisplayId) -> Option<Rc<dyn PlatformDisplay>>;
+    fn open_window(
+        &self,
+        handle: AnyWindowHandle,
+        options: WindowOptions,
+    ) -> Box<dyn PlatformWindow>;
+}