(bug 51460) Introduction of $.wikibase.claimgrouplabelscroll

Widget which will reposition labels of Claim groups while scrolling through the page. This
ensures that the labels are always displayed on the same line with the first Main Snak
visible within the viewport. When the label gets moved, the movement is animated for a smooth
transition.

Change-Id: I0639c32633342884921d7f639947ae8f419dd91d
Merge "(bug 51460) Introduction of $.wikibase.claimgrouplabelscroll"