Light node hiding / showing

- Add "Show Lights" menu item to the "Document -> Show" menu. This toggles the
hidden state of all light icons in the stage.
- Toggle hidden states of individual lights via the visibility column in the sequencer.
- Link the state of the "Show Lights" menu item to the hidden state of all lights. If all
lights are hidden, the menu item is off (checkmark hidden). Otherwise the menu item
is on (checkmark visible).
- Add willChangeValueForKey / didChangeValueForKey for showLights and showJoints
at the end of -[AppDelegate replaceDocumentData:]. This makes sure the corresponding
menu items are synchronized with state of the document after loading.
- Move the "Show Joints" menu item into the "Document -> Show" menu.