trigger willScrollToDateSegmentWith when scrolling programmatically with scrollToSegment

[elliotcz97]

This PR is a fix and a followup on #1262

My problem was that when I programmatically scrolled the calendar with the function scrollToSegment, then the willScrollToDateSegmentWith delegate did not get called.

I wanted this delegate to work because i wanted to deselect all dates before the scroll did end. I searched for an answer and found the PR #1264 which made the delegate work when the user scrolled with dragging the scrollview. But it did not fix my issue. So i forked his solution and added my fix.

So this PR fixes the willScrollToDateSegmentWith delegate. Now it will get called whenever you scroll via dragging the scrollview, or programmatically using scrollToSegment.

[bystritskiy]

You need to fix merge conflicts :)