relax SVN archive

Renamed the on_exit() wizard page method to on_execute().

Created the on_apply() wizard page method.

Fix for the ok wizard button - the method is now private.

Rearrangement of the wizard page methods and variables.

Many variables and methods which should not be touched by the derived classes have been made
private. The execute() and update() methods have been deleted. And the class docstring has been
updated to describe which methods should be over-written and which should be used.

Updated the Wiz_window class docstring.

Redesigned the wizard page so that on_exit() replaces execute().

Spacing fix in the File_selector.__init__() docstring.

The new analysis wizard page now blocks and unblocks the next button.

This is done by using the wizard page on_display() method to block, and the button toggling
unblocks.

Created the wizard method block_next() to allow a page to prevent forwards progression.

All wizard buttons are now stored in self._buttons.