CoverGenerator and its implementation for Groups and Genres

This patch contains the CoverGenerator class and its implementation for Groups and Genres. It allows us to generate 2x2 covers for Groups (divided) and 4x2 covers for Genres (duplicated and blurred).

CoverGenerator is an AysncTask (kudos @rom1v) that generates a cover from a set of thumbnails based on a MLItemId. It provides a few options like:

• Cover size, countX and countY
• Split type: divide or duplicate
• Blur and smooth resize

The next step would be to implement CoverGenerator for Playlists (which requires a few changes on the QML side).

Thanks to @chub @rom1v for their support and feedback.