Templates for creating XMP sidecar files

* xmp_sidecar.mako: Mako template for generating XMP sidecar
* xmp_sidecar_beta.mako: template for beta testing new XMP properties, if run with --beta, osxphotos will use this template, otherwise it always uses xmp_sidecar.mako

Reference: 
* https://www.adobe.com/devnet/xmp.html
* https://www.makotemplates.org/
