Tweak size and color of handles for the seek/volume sliders

Summary:
Make the handles a bit smaller and tweak the border color. This isn't breeze's proper style, but I think it fits better with the design of the player/header bar.

Also this patch makes the volume slider look disabled when muted. (see screenshot)

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Subscribers: ngraham, mgallien, astippich

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D12842