Reorganize source code for Elements; obsolete those we don't use

[CharliePoole]

Fixes #1170

[rowo360]

The Readme.md file really provides a perfect summary of the fundamental design approach. It is really beneficial that it is now written down so that everyone can take this into account in the future.
You have also listed each interface and class by name and explained its primary purpose - that's a good overview!
Of course this file should be maintained now, when adding new classes/interface to the Elements folder.

[rowo360]

Very nice - it will help to keep the design.
(Minor issue - I adapted one spelling issue in the Readme file myself)

[rowo360]

This PR is assigned to me, so I assume that I should learn to review and merge a PR. :-)
As it looks fine for me, I'll do the merge now.

[CharliePoole]

Thanks. Hopefully, having the file right there in the directory should help us keep it up to date.