Add a getter/setter for torrent announce URLs as text that can be copied
and pasted: one URL per line, with a blank line between tiers.
C API: tr_torrentGetTrackerList() and tr_torrentSetTrackerList()
RPC APi: `trackerList` in `torrent-get` and `torrent-set`.
This deprecates `trackerAdd`, `trackerRemove`, and `trackerEdit`
from the RPC API.
