move all image and icon strings to theme file

Summary:
Move all icon and image definitions to theme file and make some consistency fixes.
In anticipation of a colorized version of the artist image, there are two definitions which currently hold the same icon.
Also enables the display of a default cover in context view.

Test Plan: no icon issues throughout the app

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D9697