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
This fixes a number of small issues or inconsistencies among all the button styles and missing state visuals.
Edited by Sam Hewitt