Fixed windowing/windowExample

[FMagnani]

The behaviour was not the expected one.

The bounce of the ball on any of the 4 walls always led to an update of the window position towards positive y, making the window to steadily move towards the lower end of the screen.

This was due to the initialization of the variables describing the window position inside ofApp::update().

Now, the variables are initialized in ofApp.h and they're just updated into ofApp::update(). The behaviour is now as expected.