Add a simple 'Hello, World' plugin

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Put the Hello World plugin into a menu

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a main() method to the Hello World plugin for debugging

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Document the Hello World plugin

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