(bug 45568) propagate claim and label changes

In order to allow dispatch and handling of more types
of changes, ChangeHandler and related classes needed
some refactoring. All functionality related to handling
changes on the client side has been consolitated in
ChangeHandler.

Change-Id: I8deaf8e2b5d1e5efe56f439f3bba2693eb0bfb11
Merge "(bug 45568) propagate claim and label changes" into mw1.21-wmf12