replace use of depreciated QDesktopWidget in GUI * gui-utils.cc: include QApplication, QRect and QScreen; (get_screen_geometry): return QRect with geometry of primary screen; (adjust_to_screen): adjust given QRect to be completely on the screen that contains the largest part of the QRect, use a default geometry if no part is on an available screen * gui-utils.h: include QRect; new function get_screen_geometry and adjust_to_screen * community-news.cc: do not include QDesktopWidget, but gui-utils.h; (get_screen_geometry); move function to gui-utils.cc; * community-news.h: move get_screen_geometry to gui-utils.h * find-dialog.cc: do not include QDesktopWidget, but gui-utils.h; (restore_settings): use new function adjust_to_screen * main-window.cc: do not include QDesktopWidget, but gui-utils.h; (get_screen_geometry: move function to gui-utils.cc * main-window.h: move function get_screen_geometry to gui.utils.h; * octave-dock-widget.cc: do not include QDesktopWidget, but gui-utils.h QScreen and QWindow; (handle_settings): use adjust_to_screen and QGuiApplication->primaryScreen * release-notes.cc: do not include QDesktopWidget, but gui-utils.h and QScreen; (get_screen_geometry): move to gui-utils.cc; * release-notes.h: move get:screen_geometry to gui-utils.h * terminal-dock-widget.cc: do not include QDesktopWidget but QScreen; (terminal_dock_widget): use QGuiApplication->primaryScreen instead of QDesktopWidget->screenGeometry * welcome-wizard.cc: remove include of QDesktopWidget

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