Make knockout.list scroll to initially visible divider or index

When the visibleIndex is already set to a value on initialisation of knockout.list, this index is not actually made visible - it only subscribes so later changes get handled.

This pull request changes that, so the initial visible index is scrolled into view. If this is not desired, it can be done outside as well of course, by just pretending the value changed - but this would require knockout.list to tell when it's done initialising.