when adding a compilation by drag-n-drop, it gets sorted wrongly

[matejcik]

I think that i've seen this bug before, but i was unable to find it now. So sorry if it's a duplicate.

When you drag-drop a compilation album into a playlist, it is sorted by Artist name instead of Track number. That is obviously wrong.

The problem seems to be caused by this from xl/common.py:

# Default tags for track sorting. Unless you have good reason to do
# otherwise, use this.
# TODO: make this a setting?
BASE_SORT_TAGS=('artist', 'date', 'album', 'discnumber', 'tracknumber', 'title')

This configuration seems completely wrong to me, because it will always mess up compilations. The right sort order should probably be this:
BASE_SORT_TAGS=('albumartist', 'date', 'album', 'discnumber', 'tracknumber', 'artist', 'title')

"albumartist" support seems to be a bit flaky at the moment, but for compilations this setting helps and for regular albums it doesn't seem to break much