improve exiting GUI in Qt6 by fixing crash in variable editor * octave-qobject.cc (~base_qobject): use deleteLater for main window and delete for dick widgets * variable-editor.h: new class variable with a list of all used dock widgets for the open variables * variable-editor.cc (variable_editor::variable_editor): initialize new class variable with list of current dock widgets; (variable_editor::focusInEvent): search in list of dock widgets instead of using findChildren; (variable_editor::~variable_editor): disconnect destroyed() signals of all variable dock widgets preventing a crash when dock widgets are destroyed after variable editor, no need to delete tool bar here anymore; (variable_editor::edit_variable): add new dock widget to the list (variable_editor::variable_destroyed): search in list of dock widgets instead of using findChildren; (variable_editor::variable_focused): dito

((nil .
      ((c-file-style . "gnu")
       (indent-tabs-mode . nil)
       (fill-column . 72)))
 (change-log-mode . ((indent-tabs-mode . t)))
 (makefile-mode . ((indent-tabs-mode . t))))