Fix sync of header if shown after horizontally scrolling.
Fix sync of header if shown after horizontally scrolling.

Also add implementations of scrollTo and getScrollPosition to List if
TouchScroll isn't being used as the base.

maier49/dgrid,dylans/dgrid,kfranqueiro/dgrid,dylans/dgrid,kfranqueiro/dgrid,kfranqueiro/dgrid,maier49/dgrid,dylans/dgrid,maier49/dgrid