Conform spelling of ID to Swift API design guidelines

[idrougge]

https://www.swift.org/documentation/api-design-guidelines/#general-conventions states that

Acronyms and initialisms that commonly appear as all upper case in American English should be uniformly up- or down-cased according to case conventions:

var utf8Bytes: [UTF8.CodeUnit]
var isRepresentableAsASCII = true
var userSMTPServer: SecureSMTPServer 

This includes the initialism "ID"; one might note that it is uniformly spelt with either all uppercase or all lowercase in all Swift standard library and Apple APIs.

This PR would align Matomo, which currently uses a mix of ID and Id, with those guidelines. Fallbacks with fixits have been provided for all public symbols.

[brototyp]

Hey @idrougge, thanks for the PR.
Looks good to me and I am happy to merge it though. Just one small question.