Fix bug in graph_union() when using filtered graphs
Include 'graphml' as synonym for 'xml' file format in graph IO functions

This also clarifies the relevant documentation.
Modify default constructor of adj_list<>::edge_descriptor

Now default constructed edge descriptors are impossible to occur in
regular graphs.
Improve RNG seeding
Force inline in some key wrapper functions
Fix bug in vertex/edge filter initialization in Graph.__init__()
Fix DrawingArea origin offsets in GraphWidget()
Remove unnecessary warnings in graph_draw() with inline=True
Make adj_list<> conform to AdjacencyMatrix concept
Make UndirectedAdaptor conform to BidirectionalGraph concept
Implement perfect_prop_hash()
Make reverse_graph conform to AdjacencyMatrix
Implement VF2 subgraph isomorphism from BGL

This replaces a custom implementation of Ullmann's code.