user story:
often my understanding of a slow torrent is not exactly what Deluge has in mind, so Deluge keeps downloading a couple of slow (slow for me) torrents, whilst there is free bandwidth to start another torrent
use case:
1. I go to Deluge -> Settings
2. I check "Do not count slow torrents below.." after which a previously greyed out speed input box appears for entering the threshold speed
optional use case:
1. I go to Deluge -> Settings
2. I check "Do not count slow torrents below.." after which a appear a previously greyed out speed input box for entering the threshold speed and a "Auto" checkbox (default: checked) for selecting the old behaviour instead
3A. I enter a speed > 0, to which the previously selected Auto checkbox automatically unchecks itself
3B. I enter a speed of 0 or empty value, to which the Auto checkbox automatically checks itself
this would replace the current "Do not count slow torrents" feature, where the treshold speed is calculated from who knows where