Add draft of zero gravity game

[K-Meech]

For #19 (+ see additional info from an email thread with David on that issue)

Adds rough draft of the zero gravity game:

• A pose is shown to the player (currently a very low-res stick figure 😅 ), followed by a 3 second countdown to get in position
• The player then holds the pose for 20 seconds, gaining 5 points for every second they stay within acceptable bounds
• A line at the top of the screen tracks left-right head movement, and matches head roll (to emphasise wobbling left-right e.g. while standing on one leg). When they pass outside of the acceptable left-right bounds, the line turns black and scoring stops
• If the player goes out of range of tracking (e.g. if you stand too far back), the line will disappear and scoring will also stop

Still todo:

• I've currently only added stick figures for 3 poses. I'm waiting for David's response on an email to clarify some of the remaining positions + potentially we could add all the remaining poses once graphics are complete for them.

[thompson318]

Looks good, and plays well. Added some minor suggestions.

[K-Meech]

Thanks for the comments @thompson318 - I've made the minor changes you suggested above, so I'll go ahead and merge this now.