messageList: Allow dismissal via backspace

Currently only the delete key can be used to dismiss notifications.
Some laptops don't have delete keys, resulting in needing to hit a
modifier to trigger delete.

Plus, on those keyboards that do have delete keys they're often a
little harder to strike than backspace.

https://gitlab.gnome.org/gnome/gnome-shell/-/issues/5789

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2435>

js/ui/messageList.js

@@ -543,7 +543,8 @@ export const Message = GObject.registerClass({
         let keysym = event.get_key_symbol();
 
         if (keysym === Clutter.KEY_Delete ||
-            keysym === Clutter.KEY_KP_Delete) {
+            keysym === Clutter.KEY_KP_Delete ||
+            keysym === Clutter.KEY_BackSpace) {
             if (this.canClose()) {
                 this.close();
                 return Clutter.EVENT_STOP;