New method "getXmpMetadata()" to add special information directly to the xmp-stream

[speckyspooky]

Based on the issue #971 I tried to fix the issue by setting the missing attribute on the xmp-metadata.
But the problem is that for that the final xmp-metadate-stream is required. The usage of the setXmpMetadData() is not enough.

It would be nice to have an option to get the byte stream of the xmp-metadata with getXmpMetadata()
then I would be able to fix the issue.

According on it the dynamic enhancment of the metadata would be easier.

[asturio]

In which class are you expecting to have this method. Getters could be added without much problems.