Fix the built introspection typelib

Without this, the properties of our GObject-derived classes couldn't be
accessed.

Thanks to @cfergeau for the help.
Install a PyGI override

This makes the API more Pythonic, and thus more enjoyable to use in
Python.

We could obviously add overrides for other languages, if needed.
Allow installing the overrides for other Python versions

This makes it possible to install the overrides with Python 3, for
example:

  $ ./configure
  $ PYTHON=/usr/bin/python3 ./configure

The first line would install the overrides for the default Python
version (which is usually Python 2), while users (or distro packagers)
could install them for Python 3 with the second command.
Only do the introspection/overrides if requested

This respects the --enable-introspection parameter to the configure
script.