Add ability to drag tags

[ghost]

One of the nice features of GitX was the ability to drag tags from place to place. I miss that feature in GitX (L)

[briandw]

You can drag the tags but it crashes if I do that every time.

[cemerick]

It crashes for me half the time; otherwise, it sits and does nothing. This is the feature I miss the most.

Here's a crash log, using 0.8.4: https://gist.github.com/1875896