Rename the labels of metadata accordion panels.
Add Label with content "none" to all metadata panels, when no metadata are not
available.
Make the placing of the empty label a bit nicer.
Notify the user when no metadata are available with a simple none label in the
meta data panels.

fixes #153

Merge branch 'fixes_153' into develop
Merge remote-tracking branch 'korpling/annis/develop' into develop