display readable fallback instead of id

The menu title is either the objects headline or its name. As the name
is automatically generated when using the inplace page menu, it is not
readable or understandable. Display a default string "[no headline]" in
case it gets created inplace.

fixes #72
bugfix: remove thumbnail for blog entry

The blog entry is not a page that can be created anywhere on the website
but only below a blog. As this feature is not yet supported, the blog
entry should not show up in the object class browser.