style: Refactor the button and entry drawing css

style: Refactor the button and entry drawing css

- use a single button mixin for all buttons with a saner 'style' parameter
- rework the entry mixins to be similar to the buttons'
- clear out all unused or deprecated mixins
- create some drawing functions for focus ring and high contrast inset
- replace instances of old mixins with the new

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3109>