Use real Kirigami Separators instead of blue rectangles

Summary: Using the correct element is more sementically correct and looks better.

Test Plan:

Reviewers: VDG, Discover Software Store, apol, ndavis

Reviewed By: VDG, Discover Software Store, apol, ndavis

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27184