Implement ATK's scrollSubstringTo() and scrollSubstringToPoint().
This branch implements new a11y methods needed to scroll in the current page.
This implementation should match what is defined by the IAccessible2 API and takes Mozilla's firefox implementation as reference. It is similar to the work done for GtkTextView in gtk#1625.
The Atk version in the builder should be upgraded to make this change pass in the CI. This has been similarly discussed in gtk!796.
Fixes #1203 (closed)