In 0.3.6 which has a blank designer workspace, playing a cart does not load into designer either, VOBs (voxel object bank) can be saved and loaded of items, rooms, folders, whatever.

This VOB can be loaded which contains the objects from 0.3.5b

This VOB contains the old/uneditable internal assets in rooms to copy-paste, that may get removed/make this unusable in the future
note: carts can be loaded as a VOB, PNG image files (128px max) can be imported, GIF frames to re-import as animations