use shortcut manager for the main help menu * main_window.cc (add_action): new parameter for the signal receiver; (construct_help_menu): use add_action for the actions of the menu; (construct_documentation_menu): use add_action for the actions of the menu; (set_global_shortcuts): set/unset shortcuts * main-window.h: actions are class variables, add_action with new parameter * shortcut-manager.cc (do_init_data): initialize the shortcuts in the help menu

Things to do before making a release:

  * Update the version number and release date in configure.ac.

  * Update the NEWS file.

  * Update the NEWS-<VERSION>.html, index.in, news.in, and download.in
    files for the web site.

  * Copy tarballs to ftp.gnu.org.

  * Post a release announcement to the help@octave.org and
    info-gnu@gnu.org lists.