To make the BoundMethod classes more generic, replace direct access to
private member from Signal classes with accessors or helper functions.
This allows removal of friend statements from the BoundMethod classes.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>