Big Popup and Display Buffer weirdness

[ustk]

@Christoph-Hart The big popups need a little fixing session

• The oscilloscope is working with the left channel only. Right channel stays flat
• Resizing breaks the path
• The spectrum isn't logarithmic (well, at least, for the frequencies I can't complain, it's cool to be able to discern the harmonic ranks when developping DSP. Definitely something I'd like to bring to the external buffer as well! )

In the same vein as the later, the display buffer property UseLogarithmicFreqAxis has no effect (always true)

NumChannels property of oscilloscope does not seem to do anything