* glom/base_db.cc: get_field_value_in_database(): Only complain about
a missing key if the relationship needs one.
* glom/libglom/document/document.cc:
* glom/print_layout/canvas_layout_item.cc: set_db_data(): Show an
empty image if there is no pixbuf.
* glom/print_layout/canvas_print_layout.[h|cc]: Added
fill_with_data_system_preferences() to show details from the system
preferences, if appropriate. This is possible because we do not need a
specific record to show values from the one record in System Preferences.
add_canvas_layout_item(), on_context_menu_edit(): Call it.