window: do not reset sensitivity of menu-send in rebuild
    
    The sensitivy of "menu-send" is getting more complex in the following
    patch. Let's have the logic in a single place,
    virt_viewer_window_set_menus_sensitive().
    
    rebuild_combo_menu() is called in 2 cases:
    
    1. notify::enable-accel: there is no need to update the sensitivy of
       "menu-send"
    
    2. on construction: default to false since display == NULL. It will be
       later updated when virt_viewer_window_set_menus_sensitive(). The
       default sensitivity is covered by previous .ui patch change.
    
    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Acked-by: Victor Toso <victortoso@redhat.com>