Add most of the UI for the new room list's menu button

Incomplete implementation: buttons don't work, some text is missing, etc

src/dispatcher/actions.ts

@@ -58,4 +58,9 @@ export enum Action {
      * Focuses the user's cursor to the composer. No additional payload information required.
      */
     FocusComposer = "focus_composer",
+
+    /**
+     * Opens the user menu (previously known as the top left menu). No additional payload information required.
+     */
+    ToggleUserMenu = "toggle_user_menu",
 }