Qt Designer: Fix deprecation warning

Use QDesktopWidget::availableGeometry(QWidget *) instead
of QDesktopWidget::availableGeometry(int), fixing:

shared\previewmanager.cpp:784:102: warning: 'const QRect QDesktopWidget::availableGeometry(int) const' is deprecated: Use QGuiApplication::screens() [-Wdeprecated-declarations]
             const QRect availGeometry = desktop->availableGeometry(desktop->screenNumber(lastPreview));
                                                                                                      ^

Change-Id: Iac3a5fe4931503c42c94428ac19cd1dcbfb277aa
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>