Use of PortExtensions class

[techyian]

Make better use of the PortExtensions class. Good use is being made of the MMALCameraExtensions and all Getting/Setting of configuration specifically for the camera's ports is being routed through there. For non-camera ports, the use of mmal_port_parameter_get and mmal_port_parameter_set is being done in their respective component classes, e.g. MMALVideoEncoder, and these should be routed through PortExtensions. This will make it easier to query encoder/decoder port configs going forward.