Add support for merged / moved tags

[dennisfisch]

TagManager automatically resolves the cq:movedTo property when fetching cq:tags of a resource. This resolution also happens when calling adapTo(Tag.class) on any resource that has such a property, returning the successor tag.

I originally wanted to implement mergeTag() and moveTag() as well, but basically hit a wall when considering this operations does not only shuffle individual tags around, but also handles whole subtrees of tags and creates copies and movedTo / backlinks relationships between them.