Add ContentEditable plugin to Editor

Hi Dav,

Here is the PR which adds ContentEditable plugin to the Editor.

There is no built code added - "ant all" in loader and editor directories should produce it.

In order to test the editor manually, there is a file, called manual.html. As toolbar I used "Itsa EditorToolbar" from the Gallery, but I had to modify a bit the sources, that's why I'm not loading it from CDN. As soon as CDN gets updated, I will use it directly.

I also had unit tests working, but because of today's changes (convert Frame and ContentEditable to Plugins), I will have to update and recommit them.

I hope you will review the code, make the needed changes and suggest a way to move forward.

Thanks for all your help!