Fix #9726 Revert CenterIn() and CenterOnScreen()

... back to their previous void returning roles. AlertPosition() is used instead to
check that an alert fits within the sides of the screen and all that.

Also add another CenterOnScreen() method that takes a Screen ID
so you can center a window on another monitor that the one it is currently on
(theoretically someday anyway).

headers/os/interface/Window.h

@@ -166,8 +166,9 @@ public:
 			void				ResizeBy(float dx, float dy);
 			void				ResizeTo(float width, float height);
 
-			BPoint				CenterIn(const BRect& rect);
-			BPoint				CenterOnScreen();
+			void				CenterIn(const BRect& rect);
+			void				CenterOnScreen();
+			void				CenterOnScreen(screen_id id);
 
 	virtual	void				Show();
 	virtual	void				Hide();