TinyMCE integration: TODO

* implement generic insertAsset usecase
* implement generic insertLink usecase
* when done, move these generic usecases into the editors module
* don't return xhtml snippet to caller, define a nice JSON structure
instead. (tiny chokes on <object/>s, and it's nicer if the editor can create
the tags by itself from the raw data.)
* fix wysiwyg: some of the css styles are not properly passed to the editor
content (font-size and font-family)
* define valid xhtml elements (this is a little redundant, since the usecase
handler will validate against a schema). ideally the valid_elements are
gleaned from the schema.
* define some style classes (again, this could be gleaned from the schema).
* add onunload handler to avoid data loss if the user accidentally clicks
on a link or closes the window (confirmation dialog like bxe)
* allow multiple resource types to be edited with different valid_elements
settings. (partly done)
* allow multiple editable areas. (partly done)

DONE:

* fix table plugin (it inserts bogus attributes and uses &nbsp; regardless
of the "entities" setting being "raw", which leads to errors as the nbsp
entity is not declared in cocoon). -- cleaned up in save callback
* finish generic insertImage usecase  -- done, but needs testing.