Event generation cleanups

[David Aspinall <da+pgtrac@…>]

Event model needs cleanup/simplification to be more efficient.

• Re-engage Ahsan's PGIP-filter in SessionManager.
• Consider separating internally generated events (!ScriptingUIEvents) from PGIP events generated by incoming prover messages. Incoming prover messages can be fast and furious, and shouldn't be sent around to every control under the sun.