Rather than each game backend file exporting a whole load of

Rather than each game backend file exporting a whole load of
functions and a couple of variables, now each one exports a single
structure containing a load of function pointers and said variables.
This should make it easy to support platforms on which it's sensible
to compile all the puzzles into a single monolithic application. The
two existing platforms are still one-binary-per-game.

[originally from svn r5126]