Add an interface for a legacy hook to intercept IJ#openImage

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Give the legacy interceptor for IJ#openImage a chance to intercept

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'scifio-hook'

The purpose of this branch is to add legacy hooks to intercept ImageJ
1.x' image opening. The obviously intended first user is SCIFIO, of
course.

Coming to your local imagej-legacy clone soon, stay tuned!

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