patch nag: wrap nautilus_file_get(_existing)() to NautilusFileInfo

[Christian Neumair <chris gnome-de org>]

The patch under bug 159103 [1] adds an API for getting a
NautilusFileInfo * for a particular URI. It is required to allow simple
bookkeeping for extensions. For instance, NFS/SMB-related extensions can
call
 nautilus_file_info_get_existing (some_uri);
and add or remove keywords as the internal extension state changes (we
can't remove keywords ATM, we should also add
nautilus_file_info_remove_emblem).

It's not really clear how many extensions need
 nautilus_file_info_get (some_uri);
but it might be a good idea to also expose this one for metadata
manipulation.

[1] http://bugzilla.gnome.org/show_bug.cgi?id=159103

-- 
Christian Neumair <chris gnome-de org>