For regular files, the data stored in TrackerFileNotifier (and
TrackerFileSystem below) will soon go away after the files are
notified through TrackerFileNotifier signals. In order to avoid
later synchronous queries to check for basic data, just preserve
it as GObject qdata.

This reduces the number of queries performed at the time of
processing those files. Especially noticeable on ::file-created
events, as these queries were done regardless of Tracker knowing
these files are brand new...