Feature: Disable non-responding trackers - for selected Label group

[Papoulka]

Less important to me than being able to replace tracker lists (my other feature request).

BUT - it would be very useful to be able to select a label group and disable all non-responding trackers in that group. As a corollary feature, have the option to enable all trackers whether responding or not. By using these alternately, we could occasionally optimize the tracker list population. With hundreds of torrents in each label group this is extremely tedious to do manually.

Especially since rTorrent appears to have an elusive bug / issue leading to a crash if there are many non-responding trackers.

Thanks.