kget: fix infinite notification when all tasks finish

kget: fix infinite notification when all tasks finish

this happens because transfersChangedEvent() is executed too often,
as a workaround asign a variable that is changed just before the
notifications is emmited and reset on transfer add/remove. I'm not
sure if it has to be reset in the group actions so that may break
the notification in some cases, in any case KGet is far less
annoying now