Add support for edge and port labels

[LittleHelicase]

In some cases it would be nice to display the type of an edge or the types of each port in the graph directly. Klay supports labels on edges and ports. For edges one has to specify an label object like this:

   "labels": [{ "text": "label-text" }],

in the edge. Thx to @leMaik for this, see KLay JSON Format. Currently it is unclear how to specify the width of the label so that the layouter can use this information (e.g. wenn the label is long it should reserve more space.)

[leMaik]

From what I see in Klay's source code, it isn't possible to specify the width of labels.