possibility to hide filters in history and workspace view (bug#45428) * history_dock_widget.cc(construct): use settings file for show/hide the filter; (~history_dock_widget): save current filter state to settings file; (ctxMenu): add menu entry for showing/hiding the filter; (handle_contextmenu_filter): slot for the new menu entry * history-dock-widget.h: new slot for new menu entry, new class variables for filter and show/hide-state * workspace-view.cc(workspace_view): use settings file for show/hide the filter; (~workspace_view): save current filter state to settings file; (contextmenu_requested): add menu entry for showing/hiding the filter; (handle_contextmenu_filter): slot for the new menu entry * workspace-view.h: new slot for new menu entry, new class variables for filter and show/hide-state

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");