Add CFG

[XVilka]

@Rot127 recently implemented CFG in Rizin, it would be nice to expose it also in Cutter.

[Rot127]

Since the CFG has a node for each instruction (and after RzArch for each instruction word), we need some representation which is minimal. Otherwise the graphs get very big.
Something like a ∎ for every node, colored by its type. Green for calls, yellow for entry point of procedure, red for return etc.

Printing the whole CFG with addresses can get overwhelming quickly.