Added BWindow::ResizeToPreferred().

* B_AUTO_UPDATE_SIZE_LIMITS only really makes sense for resizable
  windows, and it only sets the minimum/maximum window size.
* ResizeToPreferred() resizes the window to its preferred size, and
  also supports height-for-width layouts.

headers/os/interface/Window.h

@@ -165,6 +165,7 @@ public:
 			void				MoveTo(float x, float y);
 			void				ResizeBy(float dx, float dy);
 			void				ResizeTo(float width, float height);
+			void				ResizeToPreferred();
 
 			void				CenterIn(const BRect& rect);
 			void				CenterOnScreen();