Move scrollbar code for gtk-3.* into scrollbar.css

This separates out the CSS rules for scrollbars, scrollbar special
cases (for scrollbars in specific contexts e.g. inside a set of nested
widgets that interact in some way) and special cases that are program
specific (such as Emacs and Firefox).

General scrollbar code goes to 'scrollbar.css'.
Special case for scrollbars in windows goes to 'scrollbar_windows.css'.
Special case for scrollbars in specific programs stays in 'adapta.css',
and should be refactored later into files for each set of program
specific overrides.

Bug: 822495
Change-Id: Ib1db52ebd706944d849819cb1582d2d53e59fd43
Reviewed-on: https://chromium-review.googlesource.com/1068302
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Tested-by: Raymes Khoury <raymes@chromium.org>
8 files changed