- id maps are not editable
- handle exceptions thrown by the file reader
- texts are always above the edges
- store a default value for all maps, so that edges and nodes created after
adding a new map receive the default value too
- create node on button release, not on click (fixes a few oddities)

 /// The namespace of LEMON

 /// \todo Some more detailed description would be nice here.

 ///

 namespace lemon {

   /// The namespace of LEMON concepts and concept checking classes

   /// \todo Some more detailed description would be nice here.

   ///

   namespace concept {}

 }