style: Rewrote app grid tile drawing functions

Sam Hewittrequested to merge
wip/snwh/improved-tile-focus-states into main
  • updated mixin for tile_button
  • dropped the overview_button mixin
  • minor changes to dash, appDisplay and switcherPopup to accommodate new stylesheet
  • new focus style for all tile buttons
  • improved high contrast style support for app grid and dash

Fixes #6907

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