1. Use proper case folding function instead of `toLower()`.
2. Use locale aware comparison instead of comparing unicode code points.
Now `a` comes before `A` which is the same as the result from QCollator. A nice side effect
is now it properly compares locale specific characters (for example `C`, `Č`).
3. Improve testing. Now the test is runnable and stable on all platforms.
PR #20208.
Now the comparison function/class should be constructed before usage.
This change also make it easier to plug in into various containers which
will require a compare function type (such as std::set).
