Support for fetching page properties

Added a convenience function for fetching
the defaultsort value.

Change-Id: I0752f1560e0e93cba0e01a9a1ea834e35f8a725c
If we already have pageproperties, use it for ItemPage.fromPage

The Wikibase Id is stored in the pageproperties, so lets use
it if we can.

Change-Id: I1a4a7d4235bd4e3717f0e6248ab90f10460f264b
Merge changes I1a4a7d42,I0752f156

* changes:
  If we already have pageproperties, use it for ItemPage.fromPage
  Support for fetching page properties