Add BuildingStoreys, currently these are represented but do nothing
Start changing building data into a more structured hierarchy

This is a transitional commit, much of the code is still ifdef'd out.
Make parts more generic by moving handle usage into them

File load/save now works again
Bring back building selection operators
Bring back wall flip and axis cycle
Bring back handle deletion
Bring back wall join/extrusion
Duplicate RETKEY keymap items for PADENTER
Make edit-child use selection
Fix handle coordinate space when importing IFC
Bring back wall joins for handles with two adjacent walls
Bring back BKE_building_copy
Bring back building edit dotzoom
Bring back wall materials
Remove unused code
Improve handle drawing
Partially bring windows back, IFC import and booleaning still TODO
Update hole IFC import
Re-enable hole booleaning
Improve transform matrices for child parts