lib: bt_object_{get,put}_ref(): accept a `const` parameter

This is the first step to make the whole API const-correct.
bt_object_get_ref() and bt_object_put_ref() are not considered to
logically modify the object: they change the reference count, but the
object's readable properties remain unchanged. Considering this, it is
simpler to have a single version of each of them accepting a const
object so as to be able to get and put const objects (eventually).

bt_object_put_ref() can have the effect of destroying/freeing the
object, just like Linux's kfree() does while accepting a `const void *`
parameter.

It is safe to cast away `const` in this library because the user only
passes opaque handles to functions (`struct bt_X *` types) pointing to
non-const objects defined by the library itself in writable memory
(usually through g_new0()).

In C++11, the equivalent of having a `const struct bt_X *` object on
which you can increment and decrement the reference count would be
having an `std::shared_ptr<const bt_X>` object: the wrapper is mutable,
but the contained object is const.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>