CompoundColumns: Add test case for programmatic sorting
Keyboard: Move contains function to util/misc for reuse
Fix #901: CompoundColumns: Fix programmatic sort arrow placement

This adds a `_findSortArrowParent` extension point to Grid, so that
CompoundColumns can extend it to add logic necessary for resolving the
correct cell to show the sort arrow in when sorted programmatically.
CHANGES.md: Note CompoundColumns programmatic sort fix