Cleanup Element class.

The Element class in nifty-core is badly in need of some basic cleanup.

Remove useless, tautological comments/javadoc. Add some useful
comments/javadoc where necessary. Improve comment/javadoc grammar,
sentence structure, and spelling.

Rename getElements to getChildren, add to addChild, and the overloaded
add method that takes an index parameter to insertChild because those
methods names are infinitely clearer. Rename these methods in all
calling classes. Also rename elements field to children.

Fix code style violations so that it matches the code style of the rest
of the project.

No actual logic is altered. All tests are passing, and
integration-testing reveals no problems.