Make MPMediaItem and MPMediaEntity consistent across platforms

[rolfbjarne]

For historical reasons, the implementation of MPMediaItem and MPMediaEntity are very different between platforms:

https://github.com/xamarin/xamarin-macios/blob/3e8381091078cac387623d6b28a82085c3052b43/src/mediaplayer.cs#L45-L275

Part of the solution might involve moving the fields to a different type (say MPMediaItemProperty): https://github.com/xamarin/xamarin-macios/blob/3e8381091078cac387623d6b28a82085c3052b43/src/mediaplayer.cs#L112-L274

but more changes will be needed. This will likely be a breaking change, so marking this issue as such.