ui: Stop blocking on plt.show()

After clicking a "Compute" button, the plot gets displayed but (in OSX at least)
the compute button is not clickable again until the plot window closed.  This
makes imposible to display multiple plots at once.

Also, the parent window is not clickable while the plot is being displayed,
making impossible to do certain operations such us playing the synthesized
sounds in the model window.

This patch fixes both problems by changing all the "plt.show()" calls to
"plt.show(block=False)"