Launcher (Linux): add only an existing icon to the .desktop file

When creating the .desktop file, we should really only refer to an
icon if it is there...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Launcher: do not special-case the location of the ImageJ 1.x icon

Wayne indicated that he actually likes ImageJ/images/icon.png better as
a location for the icon than ImageJ/ImageJ.png, so as not to confuse
users into believing that the icon is the launcher.

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