Python: Rewrite get_traceback() using Boost::Python instead of C.

* glom/python_embed/glom_python.cc: get_traceback():
  Use Boost::Python to get the traceback. This code seems more
  robust, and might be more portable across Python 2 and Python 3.
Fix the build (and tests) with Python 3

* configure.ac: Added some hints in comments.
* glom/libglom/python_embed/pygdavalue_conversions.cc:
  Do not use PyInt_Check if building with python 3,
  because python 3 does not seem to have integer types.
  Use PyUnicode_Check() instead of PyString_Check().
  This seems to be appropriate for python 2 too.
* glom/main.cc: Do not call PySys_SetArgv() if building
  with python 3 because we would need to massage the
  arguments into wchar*, and we do not know why we call
  it anyway.
* glom/python_embed/glom_python.cc: Do not call
  Py_FlushLine() if building with python 3, because it
  does not exist in python 3 and is apparently
  unnecessary.
* glom/test_pyembed.cc: USe PyUnicode_Check() instead
  of PyString_Check(). See above.
  Again, do not call Py_FlushLine() with python 3.
* glom/test_pyembed_singleline.cc: Do not call
  PyFlushLine() with python 3.

To build for Python 3, you'll need to do something like this:
export PYTHON=python3.3
  (do this before starting jhbuild, if you are using jhbuild).
configure --with-boost-python=boost_python-mt-py33
  (or autogen.sh if building from git.)
You will need to have built pygobject with Python3 support.
Remove use of Gtk::RecentAction.

* glom/appwindow.cc:
* glom/bakery/appwindow_withdoc_gtk.[h|cc]:
  Remove use of GtkRecentAction, and therefore remove the
  recent files menu. GtkRecentAction has been deprecated
  without a replacement, and their has been some suggestion
  that this is a good thing:
  TODO: Link to gtk-devel-list mail.

  This does not remove the custom recent files list in our
  initial dialog.