Better interruption semantics.

When a transition is interrupted, the interrupt event is dispatched immediately
(such as within a call to selection.interrupt). This is easier to understand and
guarantees that the interrupt event on an interrupted transition is dispatched
prior to the start event on an interrupting transition, fixing #2140.

Calling selection.interrupt repeatedly no longer cancels any scheduled (but
inactive) transitions, fixing #2141. Calling selection.interrupt when there is
no active transition now has no effect.

An interrupt event is only dispatched if the active transition is interrupted,
and not if a scheduled transition was cancelled, as when a delayed transition is
superceded by an earlier transition on the same element. These transitions are
cancelled silently, fixing #2144.

Lastly, transition event listeners now see the latest bound data, rather than
using the data that was captured shortly after the transition was scheduled.
Fixes #2142.