To handle new idea, discussion after MGNLRESTCL-74

• discuss about necessary fields (headers, cookie... above comments)
• removal of body in request api.

PR for MGNLREST-146 already addressed some of those changes;

Remaining goal is to do code cleanup:

• Ditch the old MVP pattern, flatten view/view.listeners into a simple Vaadin view
• Reconsider the form abstraction for proxy-based REST client and config-based client.