Added clickableInInactiveWindow property to parts.

This property is ignored and treated as if it's true while an editing tool is up, so you can switch from the tool palette into the window without clicking twice.
You can now 'go' to a background.

Which takes you to its first card. It should really take you to the closest card from that background in the next/previous case like HC does. Will fix later.
No longer support the optional 'new' keyword on 'create new button' etc.

Will restore it after parser fixes, right now this pushes a parameter for the "new" that we'd have to remove.
Implement first/last/previous/next background and clean up part-related code a little.
We can now reference strings as arrays where needed.

Track Leonie HEAD.