Add event listeners to Application class

[fesoliveira014]

Add virtual methods for input events on the Application class.

class Application
{
public:
  // ...
protected:
  void onResized();
  void onMouseMoved();
  void onKeyPressed();
  void onMouseButtom();
  void onMouseButtomDisplacement();
  void onFocusGained();
  void onFocusLost();
  void onQuit();
  // etc...
}

These are application specific and should be implemented by the user.