EmulatorView library cleanup

These patches update the documentation for the EmulatorView library API to match changes to the code and make some library internals package-private.  No functional changes are included.

There are a couple of questions not addressed:

* How much do we care about library API stability?  The ColorScheme change to accommodate 256-color terminals was an API break, strictly, but (at least in public) no one seems to have complained.
* EmulatorView.getURLat(): the capitalization convention doesn't match the rest of the library (should probably be getUrlAt() or getURLAt()).  Also, I have a nagging feeling that this doesn't quite fit in the public API, possibly because this is the wrong place for that functionality (shouldn't EmulatorView users be able to control linkification themselves?).  That said, I haven't put any thought into how this might be improved, I'm not sure if there are outside users who would use the functionality anyway, and if we don't care about API stability, we can always fix this later if it ever becomes a problem.
* The documentation in libraries/emulatorview/docs: should we have a pre-generated copy in the source tree, should we put it elsewhere (in the GitHub pages for the project, for instance), or should we just tell users of the library to generate it themselves?  (If we want to have a pre-generated copy around, it should be updated.)