Implementation of the "Custom Tags" Feature

[Casey Link]

> - Adding a decent Dialog for editing and displaying the Tags on the Bottom of
> the Tag editing Page (Track Details). Data would be joined via the id with the
> mysql join statement.

I think adding this ui to the existing Track Details dialog is a bad
idea..  We have discussed this extensively before, though regarding
last.fm tagging +  local labels.
Please see http://lists.kde.org/?t=123507023400003&r=1&w=2

Casey