Implemented ordering of snaks within SnakList

(bug 52391)
Implemented "move" functions in wikibase.SnakList which allow moving snaks within
the SnakList while considering the snaks being grouped by property.

Change-Id: Ic3e23827144b047038790afddf86996c3efa5c34
Merge "Implemented ordering of snaks within SnakList"