Add "Last Seen Complete" column to transfer list

This column shows when we or one of our peers last saw a complete
copy of the related torrent.

Closes #517.
Add "Last Activity" column to transfer list

This column shows the time elapsed since the related torrent has
received or sent a chunk.

Torrents that have never been active will show that an infinite amount
of time has passed, paused torrents will not show anything.

Closes #483.
Add "Total Size" column to transfer list

This column shows the total size of the related torrent taking into
account also the unwanted data.

Closes #1233.