Subassembly icon choice

[SirJodelstein]

Currently, all subassemblies have the same icon (the green rocket). This icon currently just takes up screen space without adding any information. It would be great if the user could pick an icon when saving a subassembly to make this space actually useful. I would like something to categorize my subassemblies, and currently this is really difficult without folders. The icons that would be useful here (Rover/Base/Lifter/Plane/"Stuff") at least partially already exist in the map/tracking station.

Accordingly, the list of subassemblies should be sorted by this categorization icon first, and only after that by name.