[Feature] Reuse idle windows

[jaruba]

If there is an idle Powder window (main menu), starting a new instance should just reuse the idle window instead of starting a new one.

Things to think about:

• find a secure way for the windows to communicate and expose their current states. (as we can't stop the creation of the new window)
• the idle window will not receive focus as the focus will be on the new window that we're closing (if we choose to pass the arguments to the idle window)
• would it not be easier to just reposition the new window over the old one and close the idle one? (the result of this might look less glitchy)

I'm open to ideas about this, it's a very small improvement that might be more trouble then it's worth, but i'm all for it if I find an efficient / satisfactory way to do it.