Bugfix releasing a key that was pressed over another window (i.e. Shift)

Bug: Open a Nodebox window, go to another X window, press "Shift" key, focus again on the Nodebox window with the mouse and release "Shift" key. Nodebox exits with a ValueError exception because the key is not in the _keys list of pressed keys.
Environment: Xubuntu Linux 14.04 (XFCE desktop)

Solution: ignore ValueError exceptions when removing keys from the _keys list