[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:
{F7752629}
After:
{F7752630}

Reviewers: #breeze, #vdg, ngraham

Reviewed By: #breeze, #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25246