Preparations for new BiomeMode

A new biome mode will soon be added to simulate the old pre-biomegroup
behaviour. A little bit of refactoring is useful for this.

To avoid code duplication an abstract LayeredBiomeGenerator class has
been added. NormalBiomeGenerator and FromImageBiomeGenerator now inherit
from this.

BiomeMode:FromImage is no longer handled by special cases in the biome
code. The Layer creation code has been moded to a new class,
LayerFactory.
Don't use biome group id 0

I haven't yet tried to find out why, but the id 0 is not a valid biome
group id, and causes the group to not generate at all. I suspect it's
used as a null value, instead of the often used -1.

Also removed a parameter from the isRoomForMoreGroups method, parameter
was not used.