Move episode-specific constants into EPISODE.CK?, defined at runtime

[sulix]

At the moment, a large number of constants, particularly graphics chunk numbers and sound chunk numbers, are manually switched in a episode-specific DefineConstants() function. Instead, lets make these variables. We make the main file EPISODE.CK?, which includes ACTION and STRINGS.

Still a lot to do: in some cases we need the values at compile time (or pointers to the values at compile time), which we can't do with these dynamically parsed ones, so that'll require some thought.