Assorted small stuff

[Stefan4472]

Note: individual tasks may be broken out into separate issues as needed.

• [x] Get ScoreDisplay working
• [x] Fix Asteroid Healthbars
• [x] Coins should give points
• [x] Bullet-Fired animation
• [x] Decrease bullet shoot period
• [x] Play sound on button click
• [x] "Master" debug switch
• [x] Handle activity lifecycle (game gets stuck when you completely leave)

[Stefan4472]

Activity Lifecycle topic moved to #155

[Stefan4472]

Bullet-fired animation will just be handled by #94

[Stefan4472]

Decrease Bullet Period will be handled by #156