Use Qt::GlobalColor to refer to color instead of string

Qt uses binary search to convert string to QColor, we don't need that
binary search at all. This patch could be considered as optimization, but
in reality creating QColor takes only 0.2% of time. So it should be visible
at all.

This could be considered as cleanup for not calling expensive functions
from non-expensive ones.
Remove mutable fields from TorrentModelItem

The querying of TR_STATUS doesn't affect color and icon now.
Call TorrentModelItem::state() directly in getTorrentStatusReport

There is no point in wraping/unwraping QVariant.