Ability to use TimeSeries.plot in GUIs

[felixchenier]

It would be nice to integrate KTK with GUI frameworks, that TimeSeries can be plotted directly to a GUI element. At the moment, TimeSeries.plot is a lazy wrapper to plt.plot, but I guess it wouldn't be that hard to get it more "integrable"

1. take axes as optional arguments?
2. return the axes the TimeSeries was plotted on?

[felixchenier]

I close, this is not the aim of ktk at the moment, and I'm pretty sure we can activate the GUI's figure using plt.figure() and then use TimeSeries.plot to plot in it.