Add support for CompositeFragments with dynamic creation of nested Fragments

[gsteinacker]

Currently you must know in advance, how many Fragments are nested into a composite fragment. Sometimes it might be more convenient, to configure these nested steps dynamically, based on the results of the first fragment.

For example, you might have a ContentProvider that is accessing a database to query some objects. For every object, you might want to access some external service to fetch content...