Ensure move events are only sent between start/end

[rodja]

This PR fixes a bug in ui.joystick where sometimes a on_move is called after on_end.

Reproduction:

ui.joystick(color='blue', size=50,
            on_move=lambda e: coordinates.set_text(f"{e.x:.3f}, {e.y:.3f}"),
            on_end=lambda _: coordinates.set_text('0, 0'))
coordinates = ui.label('0, 0')

When releasing the mouse while dragging, the text is often not set back to 0, 0.