Remove selection state change animations

Summary:
In general animations are nice, but animations for changed hover states are often
very distracting. Animating color and opacity changes can produce a visual
artifact whereby blue turns into gray. Furthermore, having the state be animated means
that moving the mouse rapidly over list items makes the highlight not follow the
mouse closely, which looks awkward.

This patch resolves those issues by removing the animation effects.

Test Plan:
Before:

After:

Reviewers: mgallien, Elisa, VDG, astippich

Reviewed By: Elisa, astippich

Subscribers: astippich

Differential Revision: https://phabricator.kde.org/D22307