Implement cancellation of ongoing speeches

This resolves gh-11.

also a few other goodies, including a context menu on the speech panel
Include time since last save in confirm-quit message

Resolves gh-12
Merge branch 'ft-gh-11-undostartspeech' into release-v3.2
Merge branch 'ft-gh-12-timesincesave' into release-v3.2