We are going to have to move away from GTask, at least in the short term
because it cannot provide some fundamental ownership and life-cycle
guarantees we require.

This is the start of a new IdeTask replacement which allows us to ensure
that task data, source objects, and results are finalized only within the
GMainContext attached to the task.

It also starts adding some new convenience API that we want for propagating
results between multiple tasks.