Experimental support for inserting custom data from GPX file

[CyrilSLi]

Fixes #148

Context

Inserting custom data is useful to display values computed by an external program. I have wrote some brief docs about this in docs/xml/README.md

Contributor Checklist

• [x] Review Contribution Guidelines
• [x] Agree to the terms of Developer Certificate of Origin.
• [x] Make sure all contributed code can be distributed under the terms of the GNU Licence
• [x] Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by maintainers if required.
• [ ] Unit tests created/updated (under tests) to verify logic, and approval tests for UI Widgets
• [ ] Update Examples - Under build-scripts/examples, and regenerate examples docs if required ( make doc-examples )
• [ ] Ensure that tests pass locally - this can be tricky for approval tests with maps or text though.
• [ ] Check that pre-release tests work ( make test-distribution )

[time4tea]

Sorry, not ignoring you! Been super busy with lots of things. I'll take a closer look soon. Thank you!