panel: Restrict app menu width

Window titles aren't restricted in length, so the menu may end up unwieldily
width. Commit 0bec76b6ee therefore limited the app context menus, but that
got accidentally dropped in commit 0ded0dbfd5. Add back the limitation and
extend it to the new app menu as well.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/624

data/theme/gnome-shell-sass/_common.scss

@@ -1096,6 +1096,11 @@ StScrollBar {
     .label-shadow { color: transparent; }
   }
 
+  .app-menu,
+  .app-well-menu {
+    max-width: 27.25em;
+  }
+
   .aggregate-menu {
     min-width: 21em;
     .popup-menu-icon { padding: 0 4px; }

js/ui/panel.js

@@ -78,6 +78,8 @@ class AppMenu extends PopupMenu.PopupMenu {
     constructor(sourceActor) {
         super(sourceActor, 0.0, St.Side.TOP);
 
+        this.actor.add_style_class_name('app-menu');
+
         this._app = null;
         this._appSystem = Shell.AppSystem.get_default();