pothos-plotters toolkit

[guruofquality]

Split up the qwt based plotters from pothos-widgets into a new toolkit called pothos-plotters. The pothos-widgets toolkit will only contain widgets based around Qt. These two sets of code are already largely independent. And the pothos-widgets will be more stand-alone, as the plotters toolkit will depend on backend blocks (like wave-trigger) and fft, and window functions that might be found in the pothos-comms toolkit.

[guruofquality]

Official wiki page for plotters toolkit: https://github.com/pothosware/pothos-plotters/wiki