New apple should be a new instance of Apple

An new apple should be a new instance of the Apple class,
not just the a method, of the class.
This makes it clearer that when you eat an apple, a new apple is
created, not moved to another postion.
Reorder constants.py to show what is configurable

Make it clear what is easily configurable in constants.py
A few constants can't be safely changed without changing many of the files
Merge branch 'master' into bonus_bugs
Add a new feature, bug bonuses.

Every time your length is divisible by 5 a bug, randomly chosen
from a tuple in constants.py, appears on the screen.
In the "statusbar" appears a counter that counts down from a random
number between 20 and 30.
If the counter reaches 0, the bug disappears, but if you eat the bug
before it times out, then the remaining time is added to your score.