BLayoutItem: Add RemoveSelf() convenience method.

It works analoguous to BView::RemoveSelf(), i.e. it removes itself from
the parent (layout in this case) and returns whether or not it had and
was successfully removed from said parent.

headers/os/interface/LayoutItem.h

@@ -23,6 +23,7 @@ public:
 	virtual						~BLayoutItem();
 
 			BLayout*			Layout() const;
+			bool				RemoveSelf();
 
 	virtual	BSize				MinSize() = 0;
 	virtual	BSize				MaxSize() = 0;