workspace-indicator: Exclude DESKTOP windows from window previews

While nautilus removed its desktop support a while ago in favor of an
extension, it's still possible that some external X11 desktop icon app
is used. As DESKTOP windows cannot be moved between workspaces or stacked,
and aren't perceived as regular windows, it doesn't make sense to show
them as previews in the workspace switcher.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/93

extensions/workspace-indicator/extension.js

@@ -84,6 +84,7 @@ let WindowPreview = GObject.registerClass({
     _relayout() {
         let monitor = Main.layoutManager.findIndexForActor(this);
         this.visible = monitor === this._window.get_monitor() &&
+            this._window.window_type !== Meta.WindowType.DESKTOP &&
             this._window.showing_on_its_workspace();
 
         if (!this.visible)