HaikuDepot: change window frame setting.

* Moved entirely into MainWindow.
* Moved duplicated code into separate methods.
* Resize the main window on larger screens by default, as we can make
  use of the extra space.
* Use BWindow::MoveOnScreen() instead of make_sure_frame_is_on_screen()
  as the former has more info. And is even smarter now as it can
  optionally resize windows to fit on screen.
* Center window on screen by default (ie. when there are no settings).

headers/os/interface/Window.h

@@ -170,7 +170,7 @@ public:
 			void				CenterIn(const BRect& rect);
 			void				CenterOnScreen();
 			void				CenterOnScreen(screen_id id);
-			void				MoveOnScreen();
+			void				MoveOnScreen(bool resize = false);
 
 	virtual	void				Show();
 	virtual	void				Hide();