Visibility: split graph API into public and private interfaces

Visibility: split graph API into public and private interfaces

The goal here is to avoid that a component or a graph user calls methods
that do not logically apply to itself, but should according to the API's
intention. For example, a component should not be able to remove the
ports of another component, and a graph user should not be able to
create a notification iterator directly.

Therefore the existing bt_component, bt_port, bt_connection, and
bt_notification_iterator objects are considered "public": any user can
call their API.

The user-defined methods (callbacks) now accept private versions of
those objects:

* bt_private_component
* bt_private_port
* bt_private_connection
* bt_private_notification_iterator

This ensures that only the user which is called back with those private
objects can call specific, private methods, which start with the
`bt_private_` prefix.

For example, any user can get a sink component's input port by name with
bt_component_sink_get_input_port(). This function accepts a bt_component
object parameter and returns a bt_port object. However, only the called
back user can get its private version of the port with the equivalent
bt_private_component_sink_get_input_private_port(). This function
accepts a bt_private_component object parameter and returns a
bt_private_port object. The private port API has the
bt_private_port_remove_from_component() function: thus only the owner
component of a port can remove it from itself.

Each private API has a bt_OBJ_from_private_OBJ() function which
returns a _new reference_ to the equivalent public object of a private
object to be able to call public functions.

With all this, if a user calls a private function with a public object,
the compiler emits a warning similar to:

    warning: assignment from incompatible pointer type

With GCC and Clang, you can make this warning an error with:

    -Werror=incompatible-pointer-types

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>