autodetect needed tags from wysiwyg settings

wouldn't it be nice...

if user just enables their buttons in wysiwyg settings and wysiwyg filter knows what tags to allow...

as wysiwyg filter should not be bloated
* it should provide a hook to pass in additional allowed-elements
* in settings show "which module added which allowed elements"

and a helper module would do the translation

(although this sounds like a killer feature to me, it is not high prio to me right now, more a memo-to-self)