LinksUpdate: Add functions returning changed page properties

LinksUpdate: Add functions returning changed page properties

This adds getAddedProperties and getRemovedProperties functions
to LinksUpdate. They are available only after the update, so for
extensions in the LinksUpdateComplete hook. This is useful for
example if an extension caches a page property; if the property
gets changed it may want to purge the cache.

This is similar to the getAddedLinks and getRemovedLinks
functions.

Change-Id: I0c73b3d181f32502da75687857ae9aeff731f559