Don't scroll while setting the text or selection manually

Thanks for open sourcing PSPDFTextView @steipete. It saved me quite a lot of time.

`textViewDidChangeSelection:` was causing unwanted animations that conflicted with my own. The way I see it, when I’m manually setting the text I don’t want any automatic animations or scroll changes, and I can trigger those manually if I need them.

WARNING: this pull request is not backwards-compatible, as before PSPDFTextView was scrolling to the end* when setting the text and to the end of the selection when setting it. With this commit PSPDFTextView just stays where it is.

\* Not exactly the end, as it might be missing to account the `textContainerInset`. This is most likely is a bug.