Force window to front on subsequent "open" events (REnamer, f8562dc)

Now we force the main window to the front on subsequent "open" events, which means that when you drop more files on the Dock icon the associated window becomes main/key, and its preview becomes visible (if appropriate).

I am not sure exactly why I have to force this in this way, nor why the controller's window message returns nil, but it works.

Signed-off-by: Greg Hurrell <greg@hurrell.net>