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 issue1625 in 18 minutes and 32 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Style Check
passed style-check-diff #598344

00:05:59

 
  Build
passed fedora-x86_64 #598345

00:04:49

passed msys2-mingw32 #598347
win32

00:05:51

passed release-build #598346

00:06:41

 
  Flatpak
manual flatpak-manual:demo #598348
allowed to fail manual
manual flatpak-manual:icon-browser #598350
allowed to fail manual
manual flatpak-manual:widget-factory #598349
allowed to fail manual