Fix build with recent OCaml versions (including 5.0)

[ddeclerck]

Update ocaml-top to build with recent OCaml versions (up to 5.0 included).

• use cppo instead of ocp-pp (not available on 4.11 and above)
• update to ocp-index 1.3.4 (new ~qualify argument added to LibIndex.load, while not technically needed, this is more future-proof)
• minimum OCaml version explicitly set to 4.08, because of the change to ocp-index >= 1.3.4, which itself depends on ocaml >= 4.08 (P.S. : lablgtk3 needs OCaml >= 4.05, so 4.02 was too low anyways)
• silence some unused variables warnings by prepending _
• ignore the return value of GMain.Idle.add