fine tune difficulty part 3 (final)

[blaxphoenix]

• [x] longer time before circle disappers (reverse int.)
• [x] Utils.nextLong (percentage instead of margin, don't go in negative)
• [x] flash to white before disappearing
• [x] increase circle isInBoundaryCircle() to make clicking easier when circle is small
• [x] max number of circles on screen at a time?
• [x] ask for feedback (longer testing before final version)
• [x] fine tune RangeMap values
• [x] circles come too grouped