style: Rewrote app grid tile drawing functions

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>