allow unbinding GUI shortcuts (bug #65580) * gui-settings.cc (sc_value): if shortcut is not contained in the settings, take the default shortcut, otherwise take contents of settings, even if empty (no shortcut selected for the action); (set_shortcut): use sc_value and set the return value as shortcut for the action, even if empty (shortcut): do not treat empty shortcut as special case * shortcuts-tree-widget.cc (shortcut_edit_dialog): add a clear button, set the current contents of the actual column into the line edit instead of the settings value, use lambda expression instead of separate trivial slot set_default_shortcut; (shortcuts_tree_widget): do not use settings group as now sc_value is used becuase we also want to show active shortcuts that are identical to the default value; (export_shortcuts): only write the shortcut into the settings file if it differs from the default value or if a full export into a user file is desired; (write_settings): call export_shortcuts with full flag false * shortcuts-tree-widget.h: remove slot set_default_shortcut, export-shortcuts with boolean argument if full export or not

To cite GNU Octave in publications use:

  John W. Eaton, David Bateman, Søren Hauberg, Rik Wehbring (2024).
  GNU Octave version 9.1.0 manual: a high-level interactive language for
  numerical computations.
  URL https://www.gnu.org/software/octave/doc/v9.1.0/

A BibTeX entry for LaTeX users is:

  @manual{,
    title     = {{GNU Octave} version 9.1.0 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      = {2024},
    url       = {https://www.gnu.org/software/octave/doc/v9.1.0/},
  }

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.