Fix columns sorting.

Summary:
QTreeWidgetItem in Qt5 requires overloading < operator to implement
custom sorting. Use existing compare methods to do that.

Additionally, natural sorting was implemented for strings.
This allows to sort by filenames like file_<number>.

BUG:389130

Test Plan:
Click on columns to sort them.
Columns are sorted by artist, album and track number.

Reviewers: mpyne

Differential Revision: https://phabricator.kde.org/D10130