Launcher: skip the class launcher when its .jar file is not present

It makes little sense to call the -- otherwise useful -- ClassLauncher
class when the ij-launcher .jar file is not present, as it is pretty
unlikely that said class will be found in that case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: avoid segmentation fault when there is no jars/ directory

There is no jars/ directory in the plain ImageJ 1.x case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: find ImageJ 1.x' shipped JRE

ImageJ 1.x comes in a variant where the JRE is shipped, living in
<ij.dir>/jre/. Of course, this prevents multi-platform installations
or portable applications, but let's support it nevertheless.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: allow for legacy location of ij.jar

ImageJ 1.x has ij.jar in the ImageJ root directory.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: special-case legacy .cfg files' -cp option

The option -cp is not understood by the JVM itself, but only by the Java
launcher. So we have to special-case it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: don't repeat yourself

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: fall back to ImageJ 1.x when neither Fiji nor ImageJ2 are found

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: show certain messages only in verbose mode

Both the LD_LIBRARY_PATH message and the one about re-executing with
less reserved heap space can be pretty annoying.

Introduce the --verbose option and show them only when that option was
passed to the launcher.

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