add theme-changing UI for the command window

This takes the form of another option in the Options window.
(Yes, this pollutes the JESUI.py even more. I'm willing to accept that
in the short term for getting this feature implemented.)

Along with it, some new themes were added. There are simple White on
Black and Black on White themes, as well as a pair of prototype themes
optimized for colorblind users. (The colorblind themes are not final
yet, as I'm having difficulty determining appropriate colors.)
provide additional Java options in launchers

Now, there's a triad of environment variables for adjusting the Java
options:

* JES_JAVA_HOME will always use this Java installation for JES
  (even in preference to the bundled JRE on Windows).
* JES_JAVA_MEMORY will override the -Xmx512m setting for memory.
* JES_JAVA_OPTIONS will pass options at the very end of the Java
  command line.

The documentation for how the launchers work with Java has been
improved, and a new flag was added to JESstartup.