Fix crashes in Quick Open plugin - optimized
This is an alternative to !176 (closed) as discussed in comments to that earlier merge request.
See commit messages for details. The following sentence in the last commit message "This new order is consistent with the file order in the project tree." assumes that !223 (merged) (also discussed in !176 (closed)) is merged into master soon.
Sorting paths case-insensitively in ProjectFileDataProvider
turns out to be significantly slower (see the last commit message for details). Is the case-insensitive sort order of project files in the Quick Open list important enough to justify the worse sorting performance?