In twin-view (2 monitors) dialogs appear between the 2 monitors

@landinig reports:

> Dialogs tend to open in the middle of the twin view (so half the dialog in one monitor and half in the other). This is difficult to read.
> 
> Maybe it should be better to show dialogs in the middle of the screen that holds the IJ2 main window.

Migrated-From: http://trac.imagej.net/ticket/1409
