Put code to determine default passage text in a method by itself

This avoids duplication and simplifies the createInfoPassage() method.
Make StorySettingsFrame and ImageFrame inherit directly from wx.Frame

Previously, they inherited from PassageFrame, but they didn't call the
constructor of PassageFrame and don't seem to be using any of its
methods either.
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.
Fix missing import in Replace Across Story
Fixed a few unterminated RTF control words

In an RTF control word that ends with a digit, a following space is
considered part of the control word. So to make sure spaces in the
following text are not swallowed by the control word, a space should
be output after the digit.

Reported in point 3 of this post:
  http://twinery.org/forum/index.php/topic,1503.0.html
Merge pull request #133 from mthuurne/various_cleanup

Various cleanups