TodoViewQuickAddLine: fix adding a todo using the quick add line

This should be connected to the returnPressed signal that takes no args, so
the QLineEdit one.

Apparently things broke because the signal is overriden in
TodoViewQuickAddLine, overriding siganls is always a hassle.

Found while using KOrganizer.
3 jobs for !69 with work/ahmad/quickaddline in 4 minutes and 10 seconds (queued for 1 second)
latest merge request