if the sandbox/matplotlib view controller is displayed. Its state won't be
always right; instead, use the StackViewController depth.
This fixes the following bug: when popping the sandbox/matplotlib view
controller, the first responder token was not given to the console
controller!
A SandboxController is now pushed on the StackViewController and sets the
KDIonContext::sharedContext before drawing, sets a white background and handles
user input after the Python computation to dismiss the drawing screen.
Change-Id: I51b0474365a85e845227379a16d090541fa8ded7