Fix unarchiving of BScrollView with layout

* We archive views using "managed" archives, and the children are not
attached in the BView(BMessage*) constructor, but later. So it's not
possible to find the target and scrollbars in the constructor of
BScrollView.
* Make BScrollView override AllUnarchived and find the target and
scrollbars again there. The code is slightly different as there is no
guarantee that the first child will be the target in that case. The
existing code in the constructor is preserved for non-managed archives.

headers/os/interface/ScrollView.h

@@ -34,6 +34,7 @@ public:
 
 	static	BArchivable*		Instantiate(BMessage* archive);
 	virtual	status_t			Archive(BMessage* archive, bool deep = true) const;
+	virtual status_t			AllUnarchived(const BMessage* archive);
 
 	// Hook methods
 	virtual	void				AllAttached();