Add mesh variation to plants

[Greg]

Describe the problem you're trying to solve in your project
Sometimes you want your tree to look slightly different: extra branch here, extra flower there. But for all intents and purposes it should be the same plant.

Describe a feature you consider and how it will help you
For that we can have 'mesh variation' list, from where a mesh is chosen randomly when painting. It might even have a weighted chance of appearence - to make some variations rarer than others.

Describe alternatives you've considered
An alternative is to use entirely different plants. Perfectly doable, but a pain to manage.

Describe how you imagine it would work. With code, pseudo-code, diagram or a drawing
For each LOD_Variant we can have a list of mesh variations. This could look a bit cumbersome, so maybe a custom UI element (like a spreadsheet/table) is in order.