We add a file-picker written with Rendition/React. It is activated with
the `ETCHER_EXPERIMENTAL_FILE_PICKER` environment variable. Further
customisation can be done with the `ETCHER_FILE_BROWSER_CONSTRAIN_FOLDER`
variable that takes a path and allows one to constrain the file-picker to
a folder.
Related: https://github.com/resin-io/etcher/issues/2238
Related: https://github.com/resin-io/etcher/issues/2285
Change-Type: patch
Changelog-Entry: Add electron-native file-picker component.
