DiskUsage: Ensure window opens on screen

* Use BWindow::MoveOnScreen to center the window if it opens outside of the screen frame.
* Fixes #16980.

Change-Id: Icf777aea70ed0e91f57bcd425b3dddbdcb7600df
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5180
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

src/apps/diskusage/App.cpp

@@ -90,6 +90,7 @@ App::ReadyToRun()
 	}
 
 	fMainWindow = new MainWindow(frame);
+	fMainWindow->MoveOnScreen();
 	fMainWindow->Show();
 
 	if (fSavedRefsReceived) {