(bug 44095) enhanced claim diff visualizaion including refs

- still todo: qualifiers, ranks.

squashed:

(bug 44095) visualize claim differences

This commit is a follow up to 5fc812442e19d55a7d1c2072ba8a839ce7d1f067.
It tackles some design issues and adds tests, while removing the
preliminary visualization code, which needs to be re-added.

Change-Id: I7f8645df860b1860b4016003f52e7959af685a25
Merge "(bug 44095) enhanced claim diff visualizaion including refs" into mw1.21-wmf11