File extension '.odml'

In a previous version all odml files had the file extension `.odml`. After support for `JSON` and `YAML` was added, the `.odml` file extension was no longer supported.

Some people still use the `.odml` file extension, since it makes odml xml files easier to spot.

This issue raises the question whether we could support a compromise solution by adding a secondary `.odml.xxx` file extension. This makes odml files easy to spot, but keeps the appropriate file format extension which is important e.g. when viewing xml files in a browser or automatically detecting the format via the extension.

As examples, the file extensions of all saved files would look like this:
`example.odml.xml`, `example.odml.json`, `example.odml.yaml`