relax SVN archive

The molecule.add user function GUI element is now partly functional, in that a blank window is shown.

The empty window is now being created, and all the user function windows are now cleanly destroyed
on exit.

Created the infrastructure for the molecule user function GUI elements.

Initialised the molecule user function GUI element class.

Created the UF_window base class for all user functions requiring generic windows.

Renamed build_menu_sub_item() to _build_menu_item().

Sub-menus are now supported in the main relax menu bar.

The 'User functions->molecule->add' sub-menu item has been added, but is not yet functional.

Fix for the new _create_sub_menu() method.

Shifted out the generic sub-menu building code from _user_functions() to _create_sub_menu().

Shifted the 'user function' sub-menu construction into _user_functions().

Shifted out the GUI main menu bar into its own module.