Fix content sharing from AUv3 iOS

AudioUnits of version three don’t support components added to the desktop. So to use ContentSharer on iOS AudioUnits we need a way to pass a parent component (akin to the FileChooser interface). I’ve created a pull requests that adds an optional Component *parentComponent argument to each sharing method. The iOS native implementation will use the passed component to add it self to the parent component’s peer.

I’m aware of @amethystdeceiver’s work on that topic but from an API point of view prefer passing the component directly at the call over setting a parent component on a singleton class.