Sync up BCF XML with BCF API

BCF API

✔ = Present in BCF XML
❌ = Missing in BCF XML

* https://github.com/buildingSMART/BCF-API/commit/84af26ddaa03be2dd44901e773d978aaa7e9b525#diff-ac8d0a54775f67811b0c24e33fd2fa997d59a30af88b696789daf117e3095391
https://github.com/buildingSMART/BCF-API/commit/3885f36a8b6437416db35fb367864c03f65930bf
-> Custom fields
❌

* https://github.com/buildingSMART/BCF-API/commit/d92ce0deb550173b204d7c46d83428ee09b098a0
-> Change url parameter names

* https://github.com/buildingSMART/BCF-API/commit/5cbce2ff4cac0d0854f4f40730db9988a81212df
-> Topic relations
✔ (Has already been there in BCF XML)

* https://github.com/buildingSMART/BCF-API/commit/3b6653c0301a819ebb67a97533a43596aa7d3acf
-> parent / child relations
❌ -> #390

* https://github.com/buildingSMART/BCF-API/commit/c2584b006ac90f93886c64fbdf96cb9755ebb5c6
-> Viewpoint audit info
❌ -> #389

* https://github.com/buildingSMART/BCF-API/commit/fa9aa9378ce3118d9f3fd2c224767eb54d809a4e
-> translucency
✔

* https://github.com/buildingSMART/BCF-API/commit/56599838ce5072136ce069bcaf0514193ac6489d
-> Remove BIM snippet
✔

* https://github.com/buildingSMART/BCF-API/commit/f4f5035b9c2c5f30363cc4012f6cddc580732780
-> Topic files in viewpoints
✔

* https://github.com/buildingSMART/BCF-API/commit/0f3e26677589ad086a960baa786490fd235c5abe
-> id and name properties
❌ -> We've discussed this and came to the conclusion that we don't port `id` properties separately to the XML exchange. Bringing several XML files to the same project, each having their own ids, would make combining those very complicated and you would have to maintain several external ids for the same properties. If they would not be maintained, then having the ids would not provide any benefit.
#388

* https://github.com/buildingSMART/BCF-API/commit/6ce1ee5dc915b8d550ea0771457491f8551a497a
-> markers
❌ -> #387

* https://github.com/buildingSMART/BCF-API/commit/edfde645e1a53955ad2774374fc60f909f99b8c0
-> Main viewpoint guid
✔

Just for internal tracking right now😀

Review the listed BCF-API commits and linked issues #387, #388, #389, and #390; no target files or tests are named here. First determine which missing XML changes remain in scope, then reconcile the XML specification with the agreed API behavior and verify that all applicable differences are addressed.

Written by the indexing model from the issue text.