Implement ATK's scrollSubstringTo{,Point}()

This implementation is based on gtk_text_view_scroll_to_iter() and thus shares its limitations for the sake of simplicity. It should be good enough for words or substrings that do not span over multiple lines, as discussed in #1625.

I followed the IAccessible2 API and tried to imitate what Mozilla is doing as pointed out by @sthibaul. That's why scrollSubstringToPoint() only deals with the start of the substring.

A more complex implementation could manipulate the smalller rectangle in which the given substring fits. However waiting for such implementation will not help the existing chicken/egg problem as we need an implementation to find corner cases that aren't working in order to improve the implementation :)