psSet source: Implement proper NodeProxy play/stop semantics

The EventStreamPlayer keeps running. 'stop' suppresses the output
by turning all events into rests.
Add support for custom plural forms in glossary
Continue Pbind discussion
Support events' \db key
Revisions: Introduce modularity earlier
Fix typo (dictation error)
Remove debugging .trace
Start on event keys
Add periods in early slides (where I wasn't doing it consistently)
Spacing of linebreaks: " \\" --> "\\", add \frenchspacing to printhead
Kill a stupid slide, and a heading that's redundant in article mode
Line breaks for article mode ("boot the server")