(bug 43609) 'wikibase-item' data type now uses 'wikibase-entityid' data value

* 'wikibase-item' data type definition changed to use 'wikibase-entityid' data value
* adjusted UI to deal with the data type changes
* added Jeroen's original tests

This depends on the original commit I0ffafe2b as well as several commits in the DataValues
and some commits in the Wikibase extension.

Change-Id: I36bd939f96106378fb084af50fe087ae603e74c9