Skip import from DVDs by default

[wazza]

As a playable DVD won't contain anything for import, I don't think it should be marked as a collection. This would fix two things:

• It saves time, and it saves users the trouble of remembering to navigate through the Settings menu to disable import for that collection.

• If I'm ripping my DVD in the background and forget to switch off import then I end up with two applications fighting over read operations, this causes a major hang until I can kill one of the apps.

This could also apply to iPods. It doesn't cause the same hanging issues, but still seems like a needless import.

[gbooker]

(In [813]) Don't make DVDs into collections. Fixes #280