Add bonemeal interaction with saplings

I'm creating this PR with the hope we can discuss the commit comment below.
If would be better to do:
```
if (target instanceof GlowGrowable) {
```
or something similar, with GlowGrowable being an interface, that is implemented by blocks like BlockSapling.
It could have a few methods like canGrow() and grow() so we can trigger this easily for all blocks that can grow.