Make it possible to provide custom XMP metadata

[wiz78]

The PdfMetadata class is now inheritable. PdfDocument has a new delegate that can be used to provide a different implementation of that class that creates custom metadata.

e.g.:

class MyXMPMetadata : PdfMetadata
{
	private readonly XmpCore	xmp;

	public MyXMPMetadata( PdfDocument document, XmpCore xmpCore ) : base( document )
	{
		xmp = xmpCore;

		SetupStream();
	}

	protected override byte[] GenerateXmp()
	{
		return xmp?.SerializeToRDF();
	}
}