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).
Merge pull request #352 from zas/caa_types_dialog

Display a simple dialog to select cover art types