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 2014-08-05 14 41 37](https://cloud.githubusercontent.com/assets/151042/3811332/e1c33850-1c9d-11e4-9304-7d15657194e0.png)