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

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