Fix matplotlib's param not updated in the first cell

[fehiepsi]

Background

#264 is made to remove the necessity of plt.show(). However, it causes various problems as reported at #295, jupyter/notebook#3385, ipython/ipython#11098. In addition, even with #264, we need to call plt.show() in the first cell. I think that somehow, the line matplotlib.pyplot.switch_backend(backend) causes problem. Docs of switch_backend method says that "Switching to an interactive backend is possible only if no event loop for another interactive backend has started. Switching to and from non-interactive backends is always possible." As I understand, we should not call switch_backend two times in the same cell (even if we have the same backend).

This PR tries to solve above problems by not calling activate_matplotlib(backend) but only setting matplotlib.interactive(True) in _enable_matplotlib_integration() method.

The result works well in my system. RCparams are updated in the first cell. In addition, we don't need to call plt.show in the first cell.

[fehiepsi]

Closed because backend_inline is deprecated.