Small code improvements

I propose a few small fixes, that slightly improve code readability and organization here and there.
Plus one small change in how the framework is initialized: Redux is initialized by calling `ReduxFramework::init();`  and

                if ( class_exists( 'ReduxFrameworkInstances' ) ) {
                    ReduxFrameworkInstances::get_instance();
                }

                do_action( 'redux/init' );
were moved inside the `init` method at the end of it.