Launcher: make java.home match the current JRE

When a JRE was found, we should not set the java.home property
to something completely different. We use the suffix '/jre' as a
tell-tale that the JRE is actually part of a JDK, and only in that
case do we point java.home to said JDK.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: use explicit -port7 argument only with Fiji1

Since the launcher is intended to also ImageJ 1.x, let's not try to
be too smart for our own good here.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: remember startup class' name when using imagej.ClassLauncher

We need to be able to differentiate between starting Fiji or ImageJ2.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher (Linux): write a .desktop file upon first start

Such .desktop files are required to identify the applications properly
by window managers used on Linux.

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