Keep backwards compatibility with old files

Due to an error, old devede files can't be opened with this version.
This patch fixes it.

src/devedeng/project.py

@@ -722,6 +722,8 @@ class devede_project:
         if "files" in project:
             error_list = []
             for element in project["files"]:
+                if not "element_type" in element:
+                    element["element_type"] = "file_movie"
                 if element["element_type"] == "file_movie":
                     new_file = devedeng.file_movie.file_movie(element["file_name"])
                     if (new_file.error):