Implement ATK's scrollSubstringTo()

This implementation is based on gtk_text_view_scroll_to_iter() and thus shares its limitations for the sake of simplicity.

A single offset is opportunistically picked to build the iterator needed for gtk_text_view_scroll_to_iter(). That means that substrings spanning over multiple lines or larger than the current window might not be displayed optimally after scrolling.

Partially closes #1625, the toPoint() variant has been discarded.