[gtk] (9 commits) ...Merge branch 'wip/jimmac/adjust-for-contemporary-trends' into 'master'

[Jakub Steiner <jimmac src gnome org>]

Summary of changes:

  c9807f5... Adwaita: remove edge highlights (*)
  849e3c8... Adwaita: flat scales, progressbars and level bars (*)
  1499626... Adwaita: :checked:active and checked:hover states (*)
  7a3dd41... Adwaita: round progress and levelbars (*)
  874cae1... Adwaita: flatter, lighter buttons (*)
  31a7bc7... Adwaita: fix button border logic (*)
  378ad75... Adwaita: scales border (*)
  b5cb2f7... Adwaita: update image assets (*)
  45848c5... Merge branch 'wip/jimmac/adjust-for-contemporary-trends' in

(*) This commit already existed in another branch; no separate mail sent