Generator: Renamed cBiomeGenerator's Initialize() to InitializeBiomeGen().

This will allow initializing generators that implement both cBiomeGenerator and other generators.
Added GetIniItemSet() function.

It reads a block / item description from the specified INI file value and returns as cItem; stores and uses the default if value doesn't exist.
CompoGen and HeiGen read their settings in their respective Initialize() functions.