[DEV]: Remove use of `tinyxml`

[tturocy]

We are moving away from the XML-based .gbt format - which we have never supported properly anyway - in favour eventually of better formats, especially as our graphical/visual tools evolve. We no longer need to carry around tinyxml as an internal dependency to accomplish what we need to do.

Tasks

• pygambit: When reading a .gbt file use a Python XML-reading package to extract the game.
• Port graphical interface XML file reading/writing to the wxWidgets XML classes