read/set Tree/entity variation

[Graveeater]

I am currently experimenting with a tree growth mod. The easiest approach is to create scaled down versions of existing trees and use them.
Growing works by removing the old tree and placing a larger version at the same place. But since the trees have more than one variation, which is apparently selected at random, the new tree often looks quite different.
Can we get a way to read/set a tree's graphic variation? For me it would be enough to specify the variation when placing the entity.

The work around I can think of is to split (during data phase) the trees into individual prototypes with each only one variation, but that would generate quite a lot of trees.

[Rseding91]

Added for 0.15.