link undo in main window to editor if the latter has focus (bug #44402) * file-editor.cc (editor_tab_has_focus): helper function checking whether an edit area has the focus; (copyClipboard, pasteClipboard, selectAll): use this helper function; (do_undo): new slot for the undo action triggered from the main window * file-editor.h: new helper function and new slots * main-window.cc (handle_undo_request): emit a signal for undo instead of undoing in terminal window when terminal does not have focus * main-window.h: new undo signal * octave-dock-widget.cc (octave_dock_widget): connect undo signal from main-window to new virtual slot; * octave-dock-widget.h: new virtual slot do_undo, doing nothing