When headless on OS X, also disable focus stealing

This prevents the Java icon from popping up in the Dock.

Thanks to David Marshall for this wonderful tip:
https://list.nih.gov/cgi-bin/wa.exe?A2=IMAGEJ;2f70b5e7.1307

I did not find a test case where Fiji with --headless popped in the
Dock without this change. But it still seems like a good idea to me.