Comment 1 for bug 753287

This is an aspect related to bug #675057 "Filters for Library Columns"

I think we can do it without additional controls. Just keeping the last two sorts.

Example:
* First click header Bpm -> list is sorted by bpm
* secons click header year -> list is sorted by Year, if year is equal, it is still sorted by the last sort in this case BPM
* third click header genre -> list is sorted by Genre, if genre is equal it is still sorted by Year, if Genre and Year is equal it is sorted by BMP.

In addition to that we can define default secondary sorts, for example: when sorting by Album, secondary sort should always be track number.

An advanced version could be a grouped list view like many media player have.