Make sure that the AWT class name is fiji-Main on Linux

For Linux' .desktop files, the WM class has to be set
either to the AWT Toolkit implementation's class name or
to the name of the startup class, as determined by the stacktrace:

	https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/src/solaris/classes/sun/awt/X11/XWindow.java#L279

and

	https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/src/solaris/classes/sun/awt/X11/XToolkit.java#L371

Since we start both Fiji and ImageJ2 via imagej.ClassLauncher, let's
force a different startup class name for Fiji so that Linux desktop
environments can differentiate between Fiji and ImageJ2.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Create Desktop Icon: use a better StartupWMClass value

Rather than using the XToolkit's class name (which applies to *all* Java
programs), let's use the Fiji-specific startup class name (Java sets the
WM class to both).

That way, GNOME (or any other desktop environment) can tell whether the
application is Fiji or, say, ImageJ.

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