Find links in SugarCube-style <<click>> and <<link>> macros

The <<link>> syntax is ambiguous in the 2-argument case. I checked the
SugarCube source and if the last argument is a known action keyword,
it is treated as an action, otherwise it is treated as a location.

Since these macros are SugarCube-specific, this code should probably
be moved into the header-specific Python module, once that lands.
Add special passage names and tags for SugarCube

This is a temporary solution; it would be better to make the header-
specific Python module register these.
Remove latest test build in window close handler instead of destructor

It is hard to predict when destructors run, or even if they run at all:
the documentation states "It is not guaranteed that __del__() methods
are called for objects that still exist when the interpreter exits."
  http://docs.python.org/2/reference/datamodel.html
So it is more reliable to do cleanup actions on window close.

Note that the original code failed to call the superclass destructor.