Port ocamlgraph to lablgtk3

[XVilka]

GTK 2 is being deprecated, and there is largely compatible version of OCaml bindings to GTK 3 is available - lablgtk3.

See lablgtk3 branch here: https://github.com/garrigue/lablgtk/tree/lablgtk3

[backtracking]

We are aware of lablgtk3, that we already use in other OCaml projects of us (e.g. why3).
Thanks for the pointer anyway. We should indeed port OCamlGraph at some point.

[backtracking]

At least OCamlGraph was recently split into two separate opam packages, ocamlgraph and ocamlgraph_gtk, the former not depending on lablgtk anymore. (Since version 2.0.0.)