eolian_cxx: Fix callback handling on generated wrappers

eolian_cxx: Fix callback handling on generated wrappers

Added callbacks to the event EO_EV_DEL for deleting heap-allocated
function objects. This will enforce that the allocated memory,
necessary for maintaining these callable objects alive, is freed at
least when the underlaying Eo object is destroyed.

Functions and constructor methods are now able to have multiple
callback types.

Removed some unused generators, since they become inconsistent now that
functions are required to handle multiple callback types.

Allocating callback objects in the constructor methods instead of
delaying it until the final constructor is called.

Created some generators to avoid code repetition.

Now the generator parameters_forward_to_c replicate the behavior of the
generator parameters_list. The generator parameters_list was, then,
removed since it have a less intuitive name.

Added a TODO comment regarding the behaviour of simple not translating
callbacks that are not followed by a user data pointer.

The generator parameter_type was moved from "parameters_generator.hh" to
"type_generator.hh" for more consistency and convenience.