Annotations: support the addition of meta annotations

The standard annotation creation view not supports the addition of meta
annotations, too. If the POST parameter 'meta_annotations' is used it will add
all annotations in there as meta annotations to the annotations passed along.
Options dialog: return message container on message creation
Annotations: let 'add annotations' dialog support meta annotations

A user now can show another input field in the 'add annotations' dialog to add a
meta annotation to the annotation defined before.