[Tracker] Searching metadata values explicitly

["Edward Duffy" <eduffy gmail com>]

Here's a patch for tracker-search-tool that allows the user to
explicitly search certain metadata keys, ala Google.

For example, searching for "album:Global" will just search for
Audio:Album tags, instead of everything, for 'Global'. Currently, the
seach is case sensitive (any way to make rdf seraches
case-insensitive?).

This patch includes support for key:
 title  -->  DC:Title  (this isn't working for Audio:Title)
 format  -->  DC:Format
 creator  -->  DC:Creator
 keywords  -->  DC:Keywords
 mime  -->  File:Mime  (this isn't working, while `format` does)
 path  -->  File:Path
 ext  -->  File:Ext
 artist  -->  Audio:Artist
 album  -->  Audio:Album
 track  -->  Audio:Title
 subject  -->  Doc:Subject
 author  -->  Doc:Author

Attachment: tracker-search-tool-keys.patch
Description: Text Data