Remove "disqualified" concept

Allows a non-initialised stateful component to be used as a source of
settings more than once. Instead of asserting that it was only being
used once, we assert that once you are stateful you don't get used as a
bag of settings, which is the real thing we were trying to avoid.

A lot of code ends up using StatefulComponents as a source multiple
times in a row, and before this would fail.

Patch by Ian Hickson