Reduce the public API

[carlosame]

This project has a number of public and protected fields and methods that aren't really expected to be used by downstream software. This was very common in old Java code, but nowadays the tendency is to reduce the public supported API to a minimum.

• Some public and protected methods are going to be deprecated for removal (where 'removal' means becoming package-visible), while others that are highly unlikely to be used by anyone else will have its visibility directly reduced.

• public and protected fields are going to be removed. If you use any, you should switch to the relevant getter or setter method.

This reduction of the public API is a pre-requisite for other issues like #39.