[tx] Replace UFO fontinfo.plist parsing with libxml2

[kaydeearts]

This is the cleaned up version of #1510
This is part of a larger project to replace current xml parsing code in tx with the libxml2 library.

This PR replaces fontinfo.plist parsing code with libxml2, as well as sets up the AFDKO for cross-platform compatibility with libxml2 (see CMake changes)

This should also resolve issue #1467 , for which a test has been added in tx_test.py

Checklist:

• I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[skef]

Setting aside the yet-to-be-uncommented warning messages, this all looks good to me.

[josh-hadley]

👍 LGTM, congrats on getting this all working!

[benkiel]

YAY!