Removes unused imports as reported by LGTM.
There are exceptions: `import Arch_rc` is shown as an alert, but has side effects. It's not clear what the best thing to do in those cases is, so I've left them for now.
- Can be based on an edge like normal wall
- Now has a 'host' property to embed into another object (tree only)
- Mullions have different height and width properties
- Mullions or panels can be disabled
