allow headless operation in JavaScript; -exit option suggestion

Jalview should not simply set System property java.awt.headless to "true" for the NODISPLAY aka NOGUI aka HEADLESS arg. That setting would affect any other SwingJS applications running on a page.

There are two possible headless cases in JavaScript

1) we want Jalview running, but not showing anything, so that we can send it commands through the api.

2) we want Jalview to carry out a task and exit immediately -- for example, creating and saving a PNG image of an alignment.

For (1) and (2), we need to be able to tell Desktop not to add JInternalFrames, or at least not to make them visible by not making itself visible (I think).

For (2) we need a headless mode with a System.exit(0) immediately following doMain().

I note that the doc for -nodisplay

*Run Jalview without User Interface. (automatically disables questionnaire, version and usage stats checks)*

[http://www.jalview.org/help/html/features/clarguments.html]

does not say anything about setting java.awt.headless true; it just says there will be no display.