Look for current ImageJ2 when determining whether it is present
Look for current ImageJ2 when determining whether it is present

We used the presence of ij-app.jar as a telltale whether we should be
running ImageJ2 or legacy mode. However, the current ImageJ2 has the app
in the file imagej.jar.

Noticed by Curtis Rueden.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

imagej/imagej-launcher,imagej/imagej-launcher,imagej/imagej-launcher