scylla-gdb.py: scylla generate_object_graph: make label of initial vertice bold
scylla-gdb.py: scylla generate_object_graph: make label of initial vertice bold

So it is easily identifiable. Also, generally improve the readability of
labels by moving type names into a new line.

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla