Merge MutableAppContext into AppContext

There may have been a good reason for the difference at some point, or I was
still learning Rust. But now it's just &mut AppContext vs &AppContext.

crates/live_kit_client/examples/test_app.rs

@@ -89,6 +89,6 @@ fn main() {
     });
 }
 
-fn quit(_: &Quit, cx: &mut gpui::MutableAppContext) {
+fn quit(_: &Quit, cx: &mut gpui::AppContext) {
     cx.platform().quit();
 }