Large cleanup, fixes and tweaks for playlists

Summary:
Large cleanup for all qml files regarding playlist.
Increase the button size in PlayListEntry so that it is easier
to click and set icon size.
Makes SimplePlayListEntry and PlayListEntry more similar.
Fixes display of multi-disc albums in the playlist view.
Adds accessibility to SimplePlayListEntry.
Remove warnings for SimplePlayListEntry due to missing data.

Test Plan:
playlist has buttons with full delegate height,
multi-disc albums are showing correctly for the playlist,
no more warnings for SimplePlayListEntry

Reviewers: mgallien, ngraham

Reviewed By: mgallien

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D22204