[3.9] api update (mostly js) + add support for FCKEditor in formContents js function

[json] fix json serialization for recent simplejson implementation, and test encoding of entities as with earlier simplejson implementation, iterencode internal stuff is a generated function, we can't anymore rely on the _iterencode overriding trick, so move on by stoping isinstance(Entity, dict). This is a much heavier change than expected but it was expected to be done at some point, so let's go that way.

[adapters] enhanced warning

[test] fix test broken by 5721:61d6a4caa963, and update it to proper adapters to avoid deprecation warning

[3.9] update warning message

[iprogress] move adapter to entities.adapters

fix wrong calculation of clean_session_interval

[ITree] more fixes, fix fixes

[idownloadable] depends on a new mtconverter release

[ITree] fix adapter/_done_init to handle tree_[attribute|relation], add maxlevel argument to base tree view since one side effect of the new code is that it find tree view for entity types it didn't find before, hence one may want to limit tree view depth when it wasn't necessary before

[3.9] fix tests

[web] move itree adapter to entities, it may be used outside the web interface