New options to set the creation date and the modify date through new methods

[speckyspooky]

I tried to fix the issue of #971 by creation the xmp-metadata.
But the main issue was that the creation date and the modify date must be equal with the dates of the xmp-metadata.

Currently there is no option to use setter-methods to set both dates via an given date.
With both methods it would be possible to correct the issue directly on my side.

The requriement would be that the both dates can be set directly on the document/document stream and the pdf document must not stored before on the disk before the change can be done (like PdfStamper logic).

[asturio]

Can you say specify what setter are missing. It should not be a big deal adding some.