Curde support for metadata import

I tried to guess how the editor internals should work, but
I might not have it all right.
Loading should be done in the background,
but currently isn't.
Should be `Optional.ofNullable(...)` since the `FileChooser` returns `null` if nothing was selected. I just discovered that the same error was made in `showDirectoryChooser()`. Would be great to fix both.