style: Rewrote app grid tile drawing functions

- updated mixin for tile_button
- dropped the overview_button mixin
- minor changes to dash, appDisplay and switcherPopup JS to accommodate new stylesheet
- new focus style for all tile buttons
- improved high contrast style support for app grid and dash
- renamed some style-classes
- tidied up dash stylesheet

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3017>

js/ui/dash.js

@@ -153,7 +153,7 @@ class DashItemContainer extends St.Widget {
         this.destroy_all_children();
 
         this.child = actor;
-        this.child.y_expand = true;
+        this.child.y_expand = false;
         this.add_child(this.child);
     }