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

To cite GNU Octave in publications use:

  John W. Eaton, David Bateman, Søren Hauberg, Rik Wehbring (2023).
  GNU Octave version 8.3.0 manual: a high-level interactive language for
  numerical computations.
  URL https://www.gnu.org/software/octave/doc/v8.3.0/

A BibTeX entry for LaTeX users is:

  @manual{,
    title     = {{GNU Octave} version 8.3.0 manual: a high-level interactive language for numerical computations},
    author    = {John W. Eaton and David Bateman and S{\o}ren Hauberg and Rik Wehbring},
    year      = {2023},
    url       = {https://www.gnu.org/software/octave/doc/v8.3.0/},
  }

We have invested a lot of time and effort in creating GNU Octave, please
cite it when using it.  See also 'citation pkgname' for citing Octave
packages.