Radios: add support for images

Summary:
Album images where disabled for radios. This patch adds support for images in the radio view. A field has been added to EditableMetaData to allow the user to choose the image he wants for a radio. It can be a local file or an url.

Linked to: T7567

Reviewers: #elisa, mgallien, ngraham, astippich

Reviewed By: #elisa, astippich

Subscribers: ndavis, astippich, ngraham, mgallien

Differential Revision: https://phabricator.kde.org/D23859