* faster updating of trackers combobox.
* generate trackerDisplayNames just once per torrent
* refactor: cache torrent delegate's warning emblem
* refactor: change mainwin refresh debounce to 200ms
* refactor: do not store trackers, hosts in QVariant
* refactor: don't use `virtual` when it's not needed
* refactor: faster counting torrents-matching-filter
* refactor: faster tracker handling in filterbar
* refactor: improve json parser's prealloc heuristic
* refactor: make Torrent::hasError() faster
* refactor: remove redundant speed stats collection
* refactor: remove unnecessary tor->isQueued() calls
* refactor: use unordered containers where possible
* scale favicons only once, when adding to the cache
