St/rocket launch play testing

[thompson318]

Addressing #53.

• Adds a radial progress bar around the outside of the count down code.
• Updates game logic so the timer only goes down if both gaze steady and head speed is above a set threshold.
• Adds dynamic difficulty, increasing the game time and the required gaze steadiness as more games are completed.
• Adjustments to flame speed scaling, now using a scaled flame sprite instead of a particle emitter for flames.
• Added a particle emitter for some smoke effects.

Other Things to look at:

• Do we want a scrolling background at launch? see https://github.com/UCL/AstroBalance/tree/st/launch-background-move
• Adaptive difficulty needs refining and discussing with David.
• Head speed measurement implementation will need further refinement to enable Data reporting #20, which may also impact adaptive difficulty.
• Needs better graphics, I've used the background from David, but they are space based, rather than a launch scene.

[mmcleod89]

Happy with code in this PR! I've removed the frame rate counter from my other PR for now to avoid confusion, and we can add it back in separately if we think it's useful. I'd like the graphics tweak PR to be merged into this branch before this branch goes into the main if you're happy with that @thompson318