* Experimental undo and redo Python prototype
* Simplify history, use transaction jargon, use walk for more robust serialisation
* Black file, set history size, add file reference to entity_instance constructor
* Ensure that files are always passed when entities are wrapped
* Add support for undo/redo of all project module operations
* Minor fix
* Blender to IFC mappings are now managed by Blender RNA, so they don't break on undo operations. See #1475.
* Implement undo for all attribute operations. See #1475.
* Update documentation for installation of add-on
* Revert Blender RNA approach for Blender-IFC mappings, because it didn't scale, but still fix the undo/redo object memory corruption with new "reload_linked_elements" method.
Dion Moult