Remove debugging line
Add 'waiting' status; NextActions() ignores it

The user specifies it exactly like a context, but instead of getting added to
the context list, it sets the 'waiting' property (default: False) to True.

NextActions() ignores any actions with this property set; from the GTD
TrustedSystem point of view, Waiting() is a separate Thing.
Add TrustedSystem.Waiting()