Fix for #1885 and add simple caching to the data funcs for the torrentview

* Fix for #1885 (Wrong tracker icon in torrent list)
* Moved the data functions from torrentview/listview into
  torrentview_data_funcs.py
* Added caching the current value of the cell renderer for the data functions
* Reordered if-tests in deluge.common.fsize
* Disable data funcs when column is hidden