Restore Type-property to its original meaning, add TypeEx for extended features

Type-property was extended in incompatible ways. Add a new property TypeEx that includes this "overloaded" implementation so that Type can be left as specified.

Also included:
* added some string comparison safety checks in props.c
* changed item type "audio.music" to just "music" (changed in MediaServer2Spec)
* changed string constant names so it would be obvious which are from MediaServer2Spec and which are added by dleyna-server

I'm not 100% confident that I've understood the whole architecture yet, please take a closer look.