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/breadcrumbs/src/breadcrumbs.rs

@@ -136,7 +136,7 @@ impl ToolbarItemView for Breadcrumbs {
         }
     }
 
-    fn pane_focus_update(&mut self, pane_focused: bool, _: &mut gpui::MutableAppContext) {
+    fn pane_focus_update(&mut self, pane_focused: bool, _: &mut gpui::AppContext) {
         self.pane_focused = pane_focused;
     }
 }