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.
7 jobs for scrollto in 30 minutes and 45 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed debian-meson #929922

00:07:24

passed fedora-autotools #929919

00:05:38

manual fedora-distcheck #929920
allowed to fail manual
passed fedora-meson #929921

00:07:03

manual msys2-mingw64-autotools #929924
win32-ps allowed to fail manual
passed msys2-mingw64-meson #929923
win32-ps

00:16:45

 
  Docs
passed reference #929925

00:13:59