#5827: Improve torrent files tree updating performance

Do not unnecessarily emit dataChanged signals for items which didn't change.
Cache file item indices to speedup lookup.
As a bonus, this also fixes wrong file progress display in rare cases.


git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@14345 f4695dd4-2c0a-0410-b89c-da849a56a58e