Fix Building material loading
Remove unused code
Fix building edit mode wire color
Remove out-of-date comment
Small code cleanup
Remove unused parameter
Add an optional constrain_wall to BuildingHandle

- This gets loaded from IFC "ATPATH" connections, also gets set for
  window handles

- Only one per BuildingHandle is allowed for now

- Used for clipping walls

- Not yet used for transforms
Use constrain_wall in transforms
Fix center calculation for building transform with non-cursor pivot set
Implement building border select
Use ListBase for BuildingWall's BuildingWindow

Windows get properly deleted now when the corresponding handle is
deleted, easier with a listbase.
Add BKE_building_selected_handle_count
Enable snap-to-grid in building edit mode