more improvements to playlist

Summary:
Some more improvements to the playlist. First of all, adds more animations and fixes T6295.
Fixes some scaling issues and makes Show Current Track also select the item to highlight it better.
Converts actions to use signals

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Tags: Elisa

Maniphest Tasks: T6295

Differential Revision: https://phabricator.kde.org/D9280