CKEditor and newletter_parts / richtext editing comfort

[juergmessmer]

add the previously available (tinymce) add_newsletter part button to ckeditor.

also I suggest we mark parts in newsletter specially and add a delete_part event handler if possible.
Let's make richtext editing as easy as possible

[juergmessmer]

I added the necessary code to the config file

• the parameter insertable_parts
• description of how to use it