Support more types of `Annotation.element`s.

This introduces support for the most common types of arguments for
annotations, such as
* literals: ``@Annotation("string")``
* literals with explicit argument names: ``@Annotation(value = "string")``
* multiple arguments: ``@Annotation(value = "string", number = 42)``
* array literals: ``@Annotation({ElementType.METHOD, ElementType.MEMBER})``

NB: There is a bug in javalang that prevents array literals (such as in the last example)
from being parsed correctly. Only the last element of the array literal is stored.
Display one annotation per line.

When annotation a single method with multiple annotations, this improves readability
a lot.

NB FIXME: This breaks LaTeX output since the docutils line break preserving
environment and the Sphinx environment for Java methods (and most likely any other
documentable structures) are not nestable.

TODO: Implement an option to turn this of or fix the LaTeX output.