Alphabetical sorting no matter the filter

[merb]

The library should always alphabetically sort songs no what the filter is. As an example, if rating is selected as the sorting method, the songs within each rating value should be alphabetized - songs with 1 star go a-z, songs with two stars go a-z... etc.

What do you think?

See: https://bugs.launchpad.net/mixxx/+bug/1006432