gpui: Implement support for wlr layer shell

[zeroeightysix]

I was interested in potentially using gpui for a hobby project, but needed layer shell 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

[jansol]

Turning popups like menus in Zed into separate wayland surfaces would be nice, would interact much better with the compositor e.g. when the main window is a small tile (let the menu float over adjacent windows instead of covering the entire Zed window with it). And some nice to haves such as "free" background blurs to match the main window itself :P