Add option to make collection site-independent or global

[morhi]

It happened a few times that I ran into a situation where it would have been nice to have a collection that is not attached to specific sites. For instance, this would be helpful for a collection of orders. These are kind of entries that contain no content that would need to be translated.

Some downsides of the current setup that come to my mind:

• If I don't add all websites to the collection I need to select the correct site first in the CP in order to be able to access the entries of the collection.
• When attaching all sites to a collection this could potentially lead to data inconsistencies for instance if an entry doesn't get propagated to all sites, if a new site is created and attached to the collection

My thoughts are not finished, but time is, so I'll create this issue nevertheless :D Any thoughts on this are appreciated though.