References for adding external svgs to the sheets

It works exactly the same as schedules - you add some .svg file, path to it stored in ifc and you can attach it to the sheet.

UI - https://i.imgur.com/tGbCGBb.png

Still need to add some tests for references

src/blenderbim/blenderbim/bim/module/drawing/sheeter.py

@@ -182,7 +182,7 @@ class SheetBuilder:
 
         layout_tree.write(layout_path)
 
-    def add_schedule(self, reference, schedule, sheet):
+    def add_document(self, reference, schedule, sheet):
         view_path = tool.Drawing.get_path_with_ext(tool.Drawing.get_document_uri(schedule), "svg")
         if not os.path.exists(view_path):
             tool.Drawing.create_svg_schedule(schedule)