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

Author: Galland

nodebox/graphics/context.py

@@ -3807,7 +3807,10 @@ def _on_key_release(self, keycode, modifiers):
             layer.on_key_release(self.key)
         self.on_key_release(self.key)
         self._keys.char = ""
-        self._keys.remove(keycode)
+        try:
+            self._keys.remove(keycode)
+        except ValueError:
+            pass
         self._keys.pressed = False
 
     def _on_text(self, text):