[414174] - Panel background transition color bug

Summary:
Added animation on background color change.
Removed test for background color that is not needed any more.

BUG:414174
FIXED-IN:0.9.5

Reviewers: mvourlakos

Reviewed By: mvourlakos

Subscribers: ognarb, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25392