Support iPhone music syncing to supported apps via libimobiledevice

## Problem

There is no way to drop tracks from Music in to an iPhone, even when you use apps like VOX or Doppler (which support various import methods including file provider storage), but AFAIK it doesn't for android either.

## Proposal

Use libimobiledevice to allow users to just pick an app's sync folder like you would if you dragged and dropped music into it via Nautilus. Apple doesn't let you manage iTunes music (without jailbreaking), but since there are many other premium audio apps that do I think it would be a fantastic compramise.

## Prior Art

Rhythmbox supports older iphones and ipods syncing music over USB, and as I said you can view and manage the file provider storage in Nautlus:
while viewing all apps | while viewing VOX's storage
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