gpui: Implement support for wlr layer shell (#32651)

I was interested in potentially using gpui for a hobby project, but
needed [layer
shell](https://wayland.app/protocols/wlr-layer-shell-unstable-v1)
support for it. Turns out gpui's (excellent!) architecture made that
super easy to implement, so I went ahead and did it.

Layer shell is a window role used for notification windows, lock
screens, docks, backgrounds, etc. Supporting it in gpui opens the door
to implementing applications like that using the framework.

If this turns out interesting enough to merge - I'm also happy to
provide a follow-up PR (when I have the time to) to implement some of
the desirable window options for layer shell surfaces, such as:
- namespace (currently always `""`)
- keyboard interactivity (currently always `OnDemand`, which mimics
normal keyboard interactivity)
- anchor, exclusive zone, margins
- popups

Release Notes:

- Added support for wayland layer shell surfaces in gpui

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/gpui/src/platform.rs

@@ -1216,6 +1216,10 @@ pub enum WindowKind {
     /// A window that appears above all other windows, usually used for alerts or popups
     /// use sparingly!
     PopUp,
+    /// An overlay such as a notification window, a launcher, ...
+    ///
+    /// Only supported on wayland
+    Overlay,
 }
 
 /// The appearance of the window, as defined by the operating system.