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.
Add option to disable all restrictions to CAA types

This is better since the list of types may change, adding a "do not restrict" ensure
all types are downloaded if more types are added to MB.

If restriction is enabled, then previous behavior is used (downloading only images having specified types).