[Evolution-hackers] Couple new functions for ECal/EBook

[Matthew Barnes <mbarnes redhat com>]

As part of the XDG base directory effort, I've written a couple simple
functions for libebook and libecal:

        gchar *
        e_book_get_user_cache_dir (const gchar *source_uri);
        
        
        gchar *
        e_cal_get_user_cache_dir (const gchar *source_uri,
                                  ECalSourceType source_type);

These functions return the cache directory path for the given URI, using
e_get_user_cache_dir().  They also centralize the URI-to-directory-name
mangling that I see repeated in quite a few places.

I'm polling for objections because there's a valid argument to be made
that these functions really belong in libedata-book and libedata-cal
instead.  Only reason I wrote them for the client-side libraries is
because e_cal_get_user_cache_dir() is used to figure out where to cache
ECal attachments in set_local_attachment_store().

Thoughts?  If not I'll proceed.