Add a simple Beanshell script template

This is in preparation for the script editor's imminent support for
script editor templates embedded in 3rd-party .jar files.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Discover script editor templates in META-INF/script-templates/ directories

Whenever a class path element contains the directory
META-INF/script-templates/, the script editor will enumerate all files
within and interpret the relative paths as menu paths in the Template
menu. When the user selects the respective menu entries, the respective
file will be loaded into the script editor, the base name set, and the
language chosen according to the recorded file extension.

Note: Typically, .jar files built by Maven or Ant will have extra entries
for directories. These are not strictly necessary for using the classes
(as the .class entries are accessed directly) but they are necessary for
the script template discovery to work. Old Fiji Build and older versions
of MiniMaven are known to produce incorrect .jar files; don't use them.

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