alias and subclass information in the reference mime implementation

[Matthias Clasen]

I think the reference implementation for the shared-mime-info spec
should include support for the alias and subclass information. 

The patch attached to http://freedesktop.org/bugzilla/show_bug.cgi?
id=1348 adds functions to xdgmime which allow to find out whether two
mime types are the same or subclasses of each other. 

In order to effectively read the alias and subclass information, 
update-mime-database should write cache files containing this
information as extracted from the .xml sources, like it does for 
magic and glob information already. A patch to do that is attached to
http://freedesktop.org/bugzilla/show_bug.cgi?id=1332


Regards, Matthias