[Planet Studio] Make the legacy base available in sub-structure menu

Honestly, setting up the newer-style base is painful, you gotta add launch pads, hangars, towers, etc. and it just takes time.
The legacy base seen on the original Droo is just one structure itself, everything like the launch pad, runway, taxiways, and VAB are bundled together in one simple package. I think it would save a lot of structure-building time if it was selectable and placeable in the sub-structure menu. Plus, it's nostalgic.