Add the new tree objects

The new trees are not yet added to the default settings, but people can
now use them in  the Tree resource, the Sapling setting and the /tc
spawn command.

Also renamed the Forest tree to Birch.

Also fixed a crash on TerrainMode:Default when spawning a tree using /tc
spawn.
Give biome variants unique colors

AMIDST doesn't do this, but because of BiomeMode:FromImage we have to do
this.

Also added a check for duplicate colors.