Add special "instance or subclass" mode

[vostrnad]

Fixes #13

When the traversal property is set to "instance of", a checkbox appears that changes the behavior from traversing the property "instance of" (which is rarely useful) to traversing superclasses of direct superclasses in the forward direction, and traversing subclasses to direct instances in the reverse direction (instances are connected with dashed lines to distinguish them from subclasses). Bidirectional mode is supported, though undirected isn't–right now the graph simply crashes–and neither are maximum iterations and the size property. I've also made some pretty invasive changes to the code to make this feature easier to implement, so all in all I don't think this should be merged in the current state, but I'm sharing it for anyone who might find it useful, and I'd be more than happy to implement suggestions that would make this fit for merging.