fix scroll-region on html element with overflow-scroll

[axel-paillaud]

add restoreScrollPositions() in setPage() method if we have scroll-region. Use this.page instead of page.

This is related to #1211

In line 454 : console.log(page.scrollRegions) is always undefined, and I don't know why. If we do only console.log(page), we can see the scrollRegions field inside browser inspector, but I think it was populated asynchronously ?

With this version, scroll-region work well, and scrolling items are not reset when we submit the form (for example), but this.page is kind of a cheat ?