assume the following scenario:
- mixin A declares the mandatory property p
- mixin A' extends from A
- node n has mixin A'
- we'd like to migrate/downgrade node n from mixin A' to A
currently there's no easy way of replacing the assigned mixins.
assigning A first results in a NOP since A would be redundant.
removing A' first removes the mandatory property p.
a new method setMixins(String) would allow to migrate
a node from mixin A' to A while preserving 'shared' content.
the semantics of setMixins(String) would be similar to