[xhtml] fix soup2xhtml to deal with malformed div,body and html tags which may leads to malformed return value

[source] remove debug

[entity] add .clear() for bw compat

[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