WIP migrate attribute management to partial editing with 4 new features! See #1222.

New features include:

 - New clarity between null and empty string attributes
 - Attribute editing now shows appropriate UI widgets depending on data type
 - Attribute editing for enumerations now show a dropdown
 - Attribute editing is now schema sensitive

src/ifcblenderexport/blenderbim/bim/module/attribute/edit_attributes.py

@@ -0,0 +1,13 @@
+class Usecase():
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "product": None,
+            "attributes": {}
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        for name, value in self.settings["attributes"].items():
+            setattr(self.settings["product"], name, value)