Foliage utility panel

[space928]

Much like the spline panel (#54) this would allow editing of [tree] objects and provide utilities to merge and optimise them into static objects. Object merging should be automatic for an entire tile/map and should split the tile into chunks such that LODing can still work. Ideally it would also generate atlas textures but this might be done later. It could also provide tools for replacing foliage automatically with better LODed models.