mapFromSource() didn't work as expected, when used in lessThan(), it sometimes
returns an invalid QModelIndex.
A crash is observed when filtering source model via filterAcceptsRow() in #7952,
the crash is due to endless recursive of filterAcceptsRow() & lessThan() calling
each other and mapFromSource() is the culprit of it.
* NaturalCompare now returns compare result instead of "less than" result
* Change to stable sort in GUI components
* Add Utils::String::naturalLessThan() helper function
* Use Qt::CaseSensitivity type