messageList: Highlight expanded group

The new design requires that other messages and groups are faded when the user
has a group expanded. This introduces a new GLSL shader to provide the
desired effect. The new shader is used for the already existing scroll
fade and the previous one is removed. The two fades need to work together to
ensure that resulting fade looks good.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3012>

js/ui/calendar.js

@@ -819,7 +819,6 @@ class CalendarMessageList extends St.Widget {
         this._messageView = new MessageList.MessageView();
 
         this._scrollView = new St.ScrollView({
-            style_class: 'vfade',
             overlay_scrollbars: true,
             x_expand: true, y_expand: true,
             child: this._messageView,