[TabBar] Use window color instead of button color

Summary:
If some color scheme (not Breeze) doesn't use the same button and window colors the tab sticks out.

BUG: 413311
FIXED-IN: 5.65

Test Plan:
Before:
{F7787513}
After:
{F7787516}

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: davidre, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25581