Add a Javascript API for users to extend JSesh

[rosmord]

Using Java to modify/customise JSesh requires a good deal of Computer Science knowledge. However, a significant number of users are probably able to write a bit of Javascript.

Proposing a reasonably simple API in JSesh might allow simple, yet useful customisation of the software. Nothing would
be done automatically, to avoid security issues.

The way I see it :

• a startup script could be set up by users, with access to menus and the like. It would enable one to add custom menu entries and shortcuts.
• it would be possible to load and run a javascript file for automation purposes too (like batch-processing of a JSesh file).