demo_matplotlibGUI plots superimposed

[Bernd Bertschinger]

In the demo_matplotlibGui the tick marks of plot dropped spines and plot sines are sumperimposed.
The frane should be cleared after each click to the corresponding button:

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[M. Gronle]

fixes matplotlib demo scripts, that want to re-use the figure canvas multiple times. The behaviour of matplotlib.figure.add_subplots changed from MPL 3.3 on. This fixes issue #220

→ <<cset be2e70538579>>