Display a simple dialog to select cover art types



It is much more convenient than the list widget.
The dialog includes "Check All" and "Uncheck All" buttons, as well a "Help" button.

![capture du 2014-08-06 12 53 15](https://cloud.githubusercontent.com/assets/151042/3825740/de22c70a-1d57-11e4-9bb6-7798c0b51c38.png)