- 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)

 /*!

 \page undir_graphs Undirected graph structures

 The primary data structures of LEMON are the graph classes.

 Under construction.

 */