Clean up annotation parsing, don't try to parse invalid annotations

Clean up annotation parsing, don't try to parse invalid annotations

The old parser tried to parse:
@foo: some text here (other text)
Reject this.  We strictly require another colon at the end to parse
the text in between as (option) (other option).
Futher ensure we only attempt to do option parse if we find something
that matches the strict regexp ([A-Za-z]+).  This could be tightened
further.

TODO: Have a warning for something that looks like an annotation,
but isn't.

https://bugzilla.gnome.org/show_bug.cgi?id=622659