SPQR-tree support

[Andi9904]

With these changes, the widget is able to support the OGDF-class DynamicSPQRTree. The changes are demonstrated in the notebook SPQR-Trees.ipynb.

[N-Coder]

Looks good to me! Two minor things I'd change before merging:

• the force-based layout should be enabled by default for SPQR tree, the other mode doesn't really make sense here (and made me think something wasn't working at the beginning)
• make the links between virtual dashed and maybe dark gray so that they can be differentiated from regular edges, plus they should be longer than normal edges - i.e. make their default length twice as long / attraction less strong

Things that would be nice but can also wait until after this is merged:

• give the nodes of different skeletons different colors, e.g. all parallels yellow, circles red and rigids a random shade between green and blue (to tell adjacent rigids apart)
• parallel edges should be spread out
• having cluster borders drawn in would be nice (but probably also hard to do)