proof-state-change-pre-hook happens earlier than
proof-state-change-hook, i.e. before proof-done-advancing. This should
be used to register information in the currently processed span before
proof-done-advancing classifies it.

Historically PG design was to gather these information during
proof-done-advancing (or in its hook called at the end) by just
looking at the command statement. But it is often useful to look at
the output (messages and/or prompt) to gather more accurate
information. Some of this information may be needed DURING
proof-done-advancing. Hence this early hook.

new, generic interface for priority action items, see
proof-priority-action-list and proof-add-to-priority-queue, for
asynchronously adding commands to the head of proof-action-list