Internationalize the 'messageText' in 'connection'

As requested by Peter Brandt at the railML conference in Zurich, the 'messageText' in the TT-element 'connection' should be internationalized.

That could only be achieved by changing it from an attribute into an element with the additional attribute 'xml:lang'. Anyway it may also occur more than once for different languages.

See also #96 for this issue.