Implement getWaveformData in Clip; expose RMS data in Frame

[DerGenaue]

The libopenshot-audio: AuidoSampleBuffer provides a getRMS-Method that isn't exposed by the libopenshot Frame.

This should be changed, in my opinion.

Also, together with this, a generator for waveform-data should be implemented in the Clip.cpp:
(Mostly to replace the python function get_waveform of openshot-qt)

I would propose eg.

float* Clip::GetWaveformData(int samples_per_second, int channel, int start_frame, int end_frame);

Which returns both Max and RMS values for the requested range at the requested samplerate

This Request corresponds to the one over at OpenShot/openshot-qt#2035