Improve documentation of the plugins module

[itziakos]

The plugins module still needs some love

• It is better to document Traits in HasTraits classes using autodoc compatible docstrings (see e.g. https://github.com/simphony/simphony-mayavi/blob/master/simphony_mayavi/core/cuba_data_extractor.py). In this case the trait documenter extension will make its magic and properly render them in sphinx.
• Class attributes are documented as Parameters (AddSourcePanel).
• Docstring like the AddEngineSourceToMayavi could use some love.