#1 Support for multiple open files in tabs

[kickaha]

tuxcards would be greatly improved if the user were able to open a 2nd file while leaving the original file open. There are many times when I, as a user, would like to either check an entry in another file or update an entry but am working mainly in the current file. If I could open that other file without leaving my current context, it would make tuxcards even more useable.

Ideally, this would mean supporting tabs in the UI, although opening in a separate window would still be beneficial.