chore: Fix some violations of 'needless_pass_by_ref_mut' lint (#18795)

While this lint is allow-by-default, it seems pretty useful to get rid
of mutable borrows when they're not needed.

Closes #ISSUE

Release Notes:

- N/A

crates/gpui/src/app.rs

@@ -348,7 +348,7 @@ impl AppContext {
     }
 
     /// Gracefully quit the application via the platform's standard routine.
-    pub fn quit(&mut self) {
+    pub fn quit(&self) {
         self.platform.quit();
     }
 
@@ -1004,11 +1004,7 @@ impl AppContext {
         self.globals_by_type.insert(global_type, lease.global);
     }
 
-    pub(crate) fn new_view_observer(
-        &mut self,
-        key: TypeId,
-        value: NewViewListener,
-    ) -> Subscription {
+    pub(crate) fn new_view_observer(&self, key: TypeId, value: NewViewListener) -> Subscription {
         let (subscription, activate) = self.new_view_observers.insert(key, value);
         activate();
         subscription
@@ -1016,7 +1012,7 @@ impl AppContext {
     /// Arrange for the given function to be invoked whenever a view of the specified type is created.
     /// The function will be passed a mutable reference to the view along with an appropriate context.
     pub fn observe_new_views<V: 'static>(
-        &mut self,
+        &self,
         on_new: impl 'static + Fn(&mut V, &mut ViewContext<V>),
     ) -> Subscription {
         self.new_view_observer(
@@ -1035,7 +1031,7 @@ impl AppContext {
     /// Observe the release of a model or view. The callback is invoked after the model or view
     /// has no more strong references but before it has been dropped.
     pub fn observe_release<E, T>(
-        &mut self,
+        &self,
         handle: &E,
         on_release: impl FnOnce(&mut T, &mut AppContext) + 'static,
     ) -> Subscription
@@ -1062,7 +1058,7 @@ impl AppContext {
         mut f: impl FnMut(&KeystrokeEvent, &mut WindowContext) + 'static,
     ) -> Subscription {
         fn inner(
-            keystroke_observers: &mut SubscriberSet<(), KeystrokeObserver>,
+            keystroke_observers: &SubscriberSet<(), KeystrokeObserver>,
             handler: KeystrokeObserver,
         ) -> Subscription {
             let (subscription, activate) = keystroke_observers.insert((), handler);
@@ -1140,7 +1136,7 @@ impl AppContext {
     /// Register a callback to be invoked when the application is about to quit.
     /// It is not possible to cancel the quit event at this point.
     pub fn on_app_quit<Fut>(
-        &mut self,
+        &self,
         mut on_quit: impl FnMut(&mut AppContext) -> Fut + 'static,
     ) -> Subscription
     where
@@ -1186,7 +1182,7 @@ impl AppContext {
     }
 
     /// Sets the menu bar for this application. This will replace any existing menu bar.
-    pub fn set_menus(&mut self, menus: Vec<Menu>) {
+    pub fn set_menus(&self, menus: Vec<Menu>) {
         self.platform.set_menus(menus, &self.keymap.borrow());
     }
 
@@ -1196,7 +1192,7 @@ impl AppContext {
     }
 
     /// Sets the right click menu for the app icon in the dock
-    pub fn set_dock_menu(&mut self, menus: Vec<MenuItem>) {
+    pub fn set_dock_menu(&self, menus: Vec<MenuItem>) {
         self.platform.set_dock_menu(menus, &self.keymap.borrow());
     }
 
@@ -1204,7 +1200,7 @@ impl AppContext {
     /// The list is usually shown on the application icon's context menu in the dock,
     /// and allows to open the recent files via that context menu.
     /// If the path is already in the list, it will be moved to the bottom of the list.
-    pub fn add_recent_document(&mut self, path: &Path) {
+    pub fn add_recent_document(&self, path: &Path) {
         self.platform.add_recent_document(path);
     }