Introduce entityChangers

The changers wrap RepoApi and RevisionStore for the various views.
They make the views independent of the means and intricacies of persisting
a change to an Item or Property.

Api usage in labelview, descriptionview and sitelinklistview needs to be
adapted as well.

Change-Id: Iab57c239f2477cf71198b7a94d05d231fdf2dfbf
Merge "Introduce entityChangers"