class library: freqscope gui: use start/stop instead of power, which is ambiguous
class library: GUI: plotter improvements

- plotter clips curve, to avoid misleading lines for clipped values
- plotter can print to pdf now (the p-key in keyDownAction)

A problem is still that we can't distinguish where curve really exceeds range and where it is just at the maximum.
class library: format code
class library: GUI: plotter uses better step size for zoom
class library: GUI: fix plotter clipping, so that it makes it possible to display waveforms that go to the limit.