NSApp class that does the root object handling

[mulle-kybernetik-tv]

This is obviously a sharedInstance.
Move the whole "roots" handling into NSApp. The only typical root class then is only NSApp in the runtime. Windows are then tied to NSApp and other root classes.

[mulle-nat]

Probably stupid. Better: remove roots handling except for placeholder and sharedinstance from runtime. Let users use a shared instance app class (->sharedinstance) to manage user roots instead of the runtime.

Effectively, the only roots are placeholders, sharedinstances and static strings.