[apps/code][python] ConsoleController doesn't keep any pointer/boolean to know

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!

python/port/mod/matplotlib/plot_controller.cpp

@@ -3,12 +3,10 @@
 namespace Matplotlib {
 
 void PlotController::viewWillAppear() {
-  MicroPython::ExecutionViewControllerHelper::viewWillAppear(this);
   curveView()->reload();
 }
 
 void PlotController::viewDidDisappear() {
-  MicroPython::ExecutionViewControllerHelper::viewDidDisappear(this);
   m_store->flush();
 }