Fix eye-gouging green in TorrentModel

Commit ad116edac74587ccca88cb64afa445b8ce87f6d7 introduced a unintendent
change. The active torrents are shown with color (0, 255, 0) now,
instead of (0, 128, 0).

QColor("green")        is (0, 128, 0)
QColor(Qt::green)      is (0, 255, 0)
QColor(Qt::darkGreen)  is (0, 128, 0)

This commit replaces Qt::green with Qt::darkGreen.

For references see qcolor.cpp and qcolor_p.cpp.