devel/wxformbuilder: Update 3.5.0-beta => 4.2.1, take maintainership

Changelog:
https://github.com/wxFormBuilder/wxFormBuilder/blob/v4.2.1/CHANGELOG.md

• Upstream moved to github.
• Unbundle tinyxml2.

PR: 295955
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>