save and restore previously active widget in a tabbed stack (bug #42980) * main-window.cc (focus_changed): store previous widget as predecessor if new widget is tabbed togehter with the previous one; * octave-dock-widget.cc (octave_dock_widget): initialize predecessor to 0; (set_predecessor_widget): new oublic method for setting the predecessor; (set_focus_predecessor): if predecessor is defined which means that this wigdet was tabbed with predecessor, set focus to predecessor; (make_window): call set_focus_predecessor; (closeEvent): moved here from header file, call set_focus_predecessor * octave-dock-widget.h: new public function set_predecessor_widget, closeEvent moved to octave_dock_widget.cc, new private function set_focus_predecessor (), new variable _predecessor_widget holding the previously active widget in the tabbed stack

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