Implement new dsl_pipeline_buffering_message_handler_add/remove services

[rjhowell44]

When using non-live streaming sources, the application needs to be notified of buffering-messages received on the bus. See the GStreamer stream-buffering for complete information.

A new callback typedef and callback add/remove services will be added to the Pipeline API.

Once the callback is added to the Pipeline and dsl_pipeline_play is called, the bus-watch manager will call the callback each time a buffering message is received. The application should Pause the Pipeline (if playing) when a buffering message with percent < 100% is received and resume playback once a message with percent == 100% is received.

Using Python for example.
Python example of callback to pause/play on buffering start/stop

## 
# Function to be called when a buffering-message is received on the Pipeline bus.
## 
def buffering_message_handler(source, percent, client_data):

    global buffering

    if percent == 100:
        print('playing pipeline - buffering complete at 100 % for Source', source)
        dsl_pipeline_play('pipeline')
        buffering = False

    else:
        if not buffering:
            print('pausing pipeline - buffering starting at ', percent,
                '% for Source', source)
            dsl_pipeline_pause('pipeline')
        buffering = True

New callback typedef

/**
 * @brief callback typedef for a client listener function. Once added to a Pipeline, 
 * the function will be called on receipt of a buffering message on the Pipeline bus.
 * @param[in] source name of the element or component that is the source of the message
 * @param[in] percent buffering. 100% means buffering is done. 
 * @param[in] client_data opaque pointer to client's data
 */
typedef void (*dsl_buffering_message_handler_cb)(const wchar_t* source, 
    uint percent, void* client_data);

New add/remove handler services

/**
 * @brief Adds a callback to be notified when a buffering message is received by
 * the Pipeline's bus-watcher. The callback is called from the mainloop context 
 * allowing clients to Pause non-live Pipelines while buffering. 
 * @param[in] name name of the pipeline to update
 * @param[in] handler pointer to the client's callback function to add
 * @param[in] client_data opaque pointer to client data passed back to the 
 * handler function.
 * @return DSL_RESULT_SUCCESS on success, DSL_RESULT_PIPELINE_RESULT on failure.
 */
DslReturnType dsl_pipeline_buffering_message_handler_add(const wchar_t* name, 
    dsl_buffering_message_handler_cb handler, void* client_data);

/**
 * @brief Removes a callback previously added with 
 * dsl_pipeline_buffering_message_handler_add
 * @param[in] name name of the pipeline to update
 * @param[in] handler pointer to the client's callback function to remove
 * @return DSL_RESULT_SUCCESS on success, DSL_RESULT_PIPELINE_RESULT on failure.
 */
DslReturnType dsl_pipeline_buffering_message_handler_remove(const wchar_t* name, 
    dsl_buffering_message_handler_cb handler);