Generalising functions, improving screen scaling

Further code tidying.
Made minimum button/blob physical diameter settable in one place.
Made default colour of some elements settable in initial variable
declarations.
Specifying more variables in terms of the minimum physical size of
blobs.
Scoop size is now constrained, and will not appear if the arena is too
small for it to be useful.
Several fixes/clarification of blob-removal, tallying.