New tag dialog should keep previous value

When you run dialog Add a Tag from taskbrowser, you get pre-filled tag "NewTag". You enter your tag and confirm. However, when you run the dialog again, there is again "NewTag". I think the former tag, should be pre-filled.

Use case: I have a really big task list. Every morning I would like to choose a set of them which I do that day. So I have a special tag "action-today", I go through the list, use Add a Tag dialog to schedule task for today. I would have to write "action-today" only the first time. (I don't want to use Due time for this purpose because I use it for real deadline and I want to get some task done before it. If I don't do, I'll still have the original deadline)

I have created simple patch, please get it in the trunk.