Viseme suppression

[penrods]

A few enclosure updates:

• Prevent visemes (mouth movements) from overwriting text or graphics that are being displayed on the enclosure mouth.
• The button's "mycroft.stop" is now generated on button release, and only if it wasn't held long enough to trigger the menu
• Added a little button debouncing code
• Implemented a ping message that responds with the version number
• Lots of minor changes to shrink memory footprint ("String" parameters to "const String&", etc.)

[aatchison]

LGTM