Prepare for deprecating the QDesktopWidget

QDesktopWidget is marked as obsolete in docs, but it is not yet
completely deprecated, some of its methods are still in use.

Replace uses of the following methods marked as obsolete:
- QDesktopWidget::screenNumber(QWidget*) -> QWidget::screen()
- QDesktopWidget::screenGeometry(QWidget*) -> QWidget::screen()->geometry()
- QDesktopWidget::availableGeometry(QWidget*) -> QWidget::screen()->availableGeometry()

Task-number: QTBUG-76491
Change-Id: I2cca30f2b4caa6e6848e8190e09f959d2c272f33
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>