style: New window-icon class to split it from icon-dropshadow

- new class for window icons in window picker
- move HC specific styles to this new class

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/507
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3310>

js/ui/windowPreview.js

@@ -133,6 +133,7 @@ export const WindowPreview = GObject.registerClass({
         const tracker = Shell.WindowTracker.get_default();
         const app = tracker.get_window_app(this.metaWindow);
         this._icon = app.create_icon_texture(ICON_SIZE);
+        this._icon.add_style_class_name('window-icon');
         this._icon.add_style_class_name('icon-dropshadow');
         this._icon.set({
             reactive: true,