quickly design opens a new Glade instance when one is already open

[Jo-Erlend Schinstad]

When you use quickly edit, then GEdit is opened with the right files. If you then later use quickly edit while that instance is open, then it is focused. That's great.

When you run quickly design, then a glade instance is opened. If you keep it open and then run quickly design again, then another instance is created instead of switching to the existing instance.

I'm not entirely sure where this is done, so I can't be certain, but my guess is that some code for quickly edit could be copy/pasted to fix this for quickly design as well.