Make trees generate similarly to terrain, and make a proper game seed that drives both.

[mortizzle]

[davidwengier]

Trees already have a Seed, so what needs to happen here is that GameManager needs to also have a seed, and then a tree's seed should come from a random generator that is seeded with the game seed. It's probably easiest and best to extract all of that to an IRandomNumberGenerator service that can be injected.

The game seed needs to be reset on Clear All, and eventually serialized, though that doesn't have to be part of this issue.

[ScottHolden]

#185 solves most of this, it doesn't implement a game-wide seed, but rather focuses on seed-driven entities saving their seed.