Changes
- Moved FCPXML element creation functions to XMLElementExtension.swift
- Added more XMLElement initializers
- Added new resource, library, and event methods
- Added fcpxmlString property to XMLDocumentExtension.swift
- Added description property to FCPXMLElementError enum for more descriptive error messages
- Fixed bugs in fcpxResourceList, fcpxLibrary, fcpxEvents properties
- Revised documentation