Add an API for programmatically getting a path to external storage devices

(This suggestion was triggered by a customer's desire for an easier way to get to the SD card from Android; hopefully the solution would benefit other platforms too)

Currently, programmatically finding the path to the SD card on Android requires QJniObject and QJniEnvironment: https://forum.qt.io/topic/133123/write-access-to-sd-card-in-android

It would be nice to have a simpler API to get this path without the JNI or requiring the user to pick it via the native Picker dialog.

Perhaps a new enum, QStandardPaths::ExternalDataLocation, QStandardPaths::SecondaryDataLocation, or similar? This can return, say, the path to the SD card on a mobile device (if the required permissions have been obtained), or the path(s) to the external hard drive(s) on a PC. Admittedly, these aren't very "standard" paths and would require scanning, so QStandardPaths might not be suitable.