Fixing stuff

Created by: DeityLink

Fixes #6243 Fixes #6271 (closed)

• You can no longer place something in a briefcase's fake bottom if there's already something in it.
• Removed the "on_floor" requirement to build computer frames, stools, closets, canisters and iv_drips.
• Empty donut boxes, egg boxes, and the likes, can now be unfolded back into cardboard sheets.

I also made sheet_types.dm marginally easier to read and edit.