QDesktopWidget / QScreen documentation improvements

in Qt 5.11 QDesktopWidget got deprecated in favor of QScreen. Anyhow it seems the change was only done half-way:

• QDesktopWidget documentation should link to QScreen as replacement

• Arguably QApplication::desktop should be be deprecated, too, and link to QGuiApplication::screens as replacement

The QScreen documentation furthermore doesn't show how to actually get hold of an instance of QScreen:

• QScreen overview documentation should link to QGuiApplication, in particular QGuiApplication::screens and QGuiApplication::primaryScreen