add onImageSnapped, onSequenceAcquisitionStarted, onSequenceAcquisitionStopped events

[tlambert03]

These events would make it easier for the application side to "know" that a new image buffer is available to display.

As with most callbacks here, I'm aware that this implementation is slightly leaky (in that some devices may snap/start/stop without these events being emitted). That's certainly less than ideal, but it's still better than not knowing at all, and covers cases where the core API itself is used to snap/start/stop

[marktsuchida]

TODO:

• Check that MMStudio has no issues with MMCoreJ built with this (it shouldn't)
• Bump MMCore minor version at time of merge (not major since this should be source-compatible)