[GTK3] Modify scrollbar states to better resemble Breeze Qt
Summary:
Scrollbars have the following changes:
- No backdrop state (this matches Qt's lack of changes for idle scrollbars on backdrop windows)
- Make scrollbar handle use 0.5 opacity of window text colour like Breeze Qt
BUG: 413498
FIXED-IN: 5.17.3
Test Plan:
Before:
After:
Reviewers: Breeze, VDG, ngraham
Reviewed By: Breeze, VDG, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25246