Add a plugins directory feature.

 - Add a plugins directory to the installation directory where users (or administrators) can add jar files with plugins and extensions.
 - Added a button to the settings panel that opens Explorer (or the platform directory browser) in the plugins directory.
 - Obviously also added it to the launcher classpath. This was the hard part, because we wanted to put the directory under $APPDATA, but this could not be made to work with the current version of Install4j.