Refactor styling for buttons to prepare for rewrite.

This moves button styling into buttons.css for gtk-3.0 and gtk-3.22.
It should not make semantic changes to the CSS.

The following CSS classes are effected:
 - button
 - path-bar button
 - buttonbox
 - combobox
 - .linked buttons

Bug: 822495
Change-Id: I66502d4e5c124ae9d0315a1c06b110ea5391b739
Reviewed-on: https://chromium-review.googlesource.com/1112888
Commit-Ready: Josh Pratt <jopra@chromium.org>
Tested-by: Josh Pratt <jopra@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
8 files changed