Show bubble when user taps the screen

Bubble will be static and its size random
Check before creating new bubble

Don't create it, if the user taps on an already existing
bubble.
When user taps on a bubble, remove it
Rename bubble intersection method, update TODO items
Implement bubble movement

Make bubbles move when created
Remove them when they go out of the screen
Play sound if the user pops a bubble
Deflect a bubble if the user flings it