Decorative lamp lantern (fixed geometry)

Fixed geometry of the center and top parts because the author couldn't be bothered to check if models are printable/assemblable. Also removed all except one connecting pieces for the top part (to be able to position 4 of them individually on the build plate) and gave part files comprehensible names.

####Center part changes
- Added missing vertices and fixed normals for the structure meant to hold side panels
- Removed cavities from individual meshes
- Removed holes in meshes
- Unified all 13 individual meshes into a single non-selfintersecting mesh suitable for 3D printing

####Top part changes
- Added missing vertices for the slots for the connecting pieces, fixed length of one of the slots
- Removed cavities
- Removed holes in meshes