Find items in tree model and match patterns in subdirectories

Fix crash reported by @coreyberla on matrix. Also fix the pattern selection not working on expanded subdirectories, reported by Corey during the same conversation. See commit messages for details.

I've chosen to preseve the performance optimizations from !1361 (merged), instead of going back to the linear search. This required adding a lot of new code for hierarchy-aware sort order comparison, but on the other hand this is a completely self-contained code and I've tried to use descriptive symbol names and assert assumptions explicitly. Suggestions welcome to improve the clarity of this code further.