Change all action callbacks so they can be invoked by a GAction:

- add GimpActionCallback typedef:
  void (* cb) (GimpAction*, GVariant*, gpointer)
- change all action callbacks to the GimpActionCallback signature
- add "gimp-activate" and "gimp-change-state" signals to GimpAction,
  with the same signature as the resp. GAction signals
- remove all other custom action signals and only use the new
  GimpAction signals
- pass around appropriate GVariants containing booleans, int32,
  strings
- badly hack around to force a GimpProcedure pointer into a
  uint64 variant
- remove all G_CALLBACK() casts from all action callbacks,
  they all have the same signature now