Try to dispose IJ1 more nicely

Niko Ehrenfeuchter reported a bug that this developer cannot reproduce
but suspects to have something to do with a mutual block between
the event dispatch thread and the thread trying to dispose of ImageJ
1.x's windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'unblock-edt-on-quit'

This avoids blocking the Event Dispatch Thread on exit when the legacy
service wants to release images.

Apparently noticed by many, but only reported by Niko Ehrenfeuchter.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>