relax SVN archive

Shifted the prompt ps1, ps2 and ps3 variables into the 'interpreter' module.

This is the only place they are used, so they are better there.

Eliminated the defunct User_fn_class base class.

The Sequence GUI elements now default to not being read-only.

Fix for the pipe editor - this window is now the parent for the user functions it calls.

Therefore the pipe editor no longer disappears behind the main relax window.

Fix for the 'Help->Licence' menu entry.

The GPL text from the gpl() function docstring is now printed, as this is no longer a user function.

Fix for the 'Tools->System information' menu entry.

The frq user function class submenu mnemonic is now 'f', conforming to the other user function classes.

Improved the keyboard mnemonics for the user function sub-menus.

There are no longer any clashing letters.

The user function menu has been split in half as it was far too long.

The spin viewer window is now using the new build_uf_menu() function for its menus.