Say, Pi v1.6.8 - Speed and stability improvements

What's Changed

Minor Performance Improvements and Bug Fixes.
Animations will run more smoothly on Firefox and older devices.
When interrupting Pi on Firefox, the last part of Pi's speech will no longer be transcribed as user speech.

• Caching generated values of call button glow colour, and refactored this into a GlowColorUpdater class. Also added a "will-change" css property for immersion mode rectangles in Firefox to suggest hardware acceleration for transform animations. by @johncrawley in #125
• Fix for Issue #127 in package-extension.sh by @johncrawley in #129
• Added separate method to create settings button for immersive mode by @johncrawley in #131
• Interrupting Pi without recording Pi by @rosscado in #132

Full Changelog: v1.6.7...v1.6.8