Selecting multiple files / directories is not actually implemented.

Nobody has claimed this yet.

A bug I ran into just now:

I currently have:

```
let dialog_options = FileDialogOptions::new()
.select_directories()
.multi_selection();

let window = WindowDesc::new(build_schauglas)
.menu(MenuDesc::new(LocalizedString::new("menubar")).append(
MenuDesc::new(LocalizedString::new("file").with_placeholder("File")).append(
MenuItem::new(
LocalizedString::new("open").with_placeholder("Open"),
Command::new(SHOW_OPEN_PANEL, dialog_options),
),
),
));
```

And in my delegate, I am simply printing out whatever command gets in:

```
-> bool {
debug!("Cmd: {:?}", cmd);
if let Some(file_info) = cmd.get(OPEN_FILE) {
return true;
}

false
}
```

this gives me the following outputs:

- When selecting a single directory: `Cmd: Command { symbol: "druid-builtin.open-file-path", payload: Any }`
- When selecting multiple:

Info:
- Windows 10
- Druid 0.6

Start by tracing FileDialogOptions::multi_selection() through the SHOW_OPEN_PANEL command and the delegate's OPEN_FILE handling, using the single-directory and multiple-selection examples in the issue. Check where the selected paths should be emitted and verify that selecting multiple files or directories produces a command containing all selections rather than no command.

Written by the indexing model from the issue text.