Support for Plone's new Collection type

Hi! 
I've made slight changes to adapt to the Collection type while keeping all the support for the Topic type. Not much changes actually. Just added the type to the allowed types and removed the hard slice on 7 items that causes an attribute error on the new type. This limit of 7 items is also too low imho. There are many cases where users will want more than that and the behaviour is not clear or explained anywhere. It is a cleaner fail when a user adds a collection with too many items and easily understands why the slideshow does not look good than if they add a collection with 10 items and the slideshow shows only 7 for no apparent reason.

The proper way to impose a limit on the number of items would be to add one more configuration to the carousel and then limit the query to it.