Open file/folder dialogs

[fcollonval]

Describe the enhancement

Similarly to #6326, it will be nice to have helper functions to select a file or a folder. I propose to mimic qt helper functions: https://doc.qt.io/qt-5/qfiledialog.html#static-public-members.

const result = await getOpenFileName();
if(result.button.accept){
  let files = result.value;
}

const result = await getExistingDirectory();
if(result.button.accept){
  let folders = result.value;
}

[blink1073]

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