Refactor Brick initialiser

[wkeeling]

The Brick initialiser can take the BrickColour and a "score multiplier". It need not take a "destroy_after" as this can be inferred from the BrickColour. The "score_multiplier" is basically the round number.

Refactor round code accordingly and remove the conditionals that set the destroy_after value.

[wkeeling]

This has been implemented. The brick init now takes the BrickColour and round number (plus the powerup class). The destroy_after attribute has been dropped.