Create a BWindow::AlertPosition() method and use it

...to position alert's and open/save dialogs nicely inside of the parent window,
or if that is unavailable, the screen frame.

AlertPosition() is private (for now) but BAlert and BFilePanel are BWindow's friends so
BWindow allows those classes to touch it's privates.

headers/os/interface/Window.h

@@ -283,6 +283,7 @@ private:
 	struct unpack_cookie;
 	class Shortcut;
 
+	friend class BAlert;
 	friend class BApplication;
 	friend class BBitmap;
 	friend class BView;
@@ -303,6 +304,7 @@ private:
 
 	virtual	void				task_looper();
 
+			BPoint				AlertPosition(const BRect& frame);
 	virtual BMessage*			ConvertToMessage(void* raw, int32 code);
 
 			void				AddShortcut(uint32 key, uint32 modifiers,