ObjFW  Changes To API Review for 1.0

* ✅ OFXMLComment
* ❌ OFXMLElement
* ❌ OFXMLElementBuilder
* ❌ OFXMLElement (OFSerialization)
* ❌ OFXMLNode
  * Should `-[XMLStringWithIndentation:level:]` be private?
* ❌ OFXMLParser
  * Needs rename: `namespace` to `namespaceURI` to avoid problems in C++
  * Needs to parse PI into target and text
* ❌ OFXMLProcessingInstructions
  * Needs a property to access the PI - `-[stringValue]` is just `@""`
  * Needs to have target and text, `-[stringValue]` is just `@""`
* ❌ OFZIPArchive
* ❌ OFZIPArchiveEntry

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