A lot of low level code was using message_warning() and friends.
While convenient this had multiple issues:
 - message boxes were popped up asynchronous with the task
 - a single task could create multiple dialogs
 - the messages were loosing context (or did not gain it)
 - the classification had to be done by the callee
   (if it's an error, warning or info depends on the caller)
 - message_*() was also used for programmers errors

ToDo:
 check all dia_context_release() to do sth useful with the content
  - decide between warning and error by return value of the
    function taking the context
  - maybe do some in display showing like gedit (for import & drop)

 convert also the save/export stuff to use DiaContext
  - finally activate autosave in thread
  - get rid of dia_message_filename()
  - get rid of strerror() use - it had utf-8 issues