On macOS it is possible to copy from a PDF a subset of a document,
which will be transported as a new PDF document containing the
subset.
LibreOffice didn't support PDF as a valid clipboard format and
previously it also didn't support showing PDFs inside the document,
so in such cases it copy-pasted a low resolution bitmap. The result
wasn't good.

As we are now able to display PDF documents as Graphic in LO, we
can also support this use-case. This adds support for the PDF
documents as a clipboard format in general and to the macOS
backend. This commit only adds support for Writer.

Change-Id: Ib982b55391b390ae06974b4ad836e376dd722a4c
Reviewed-on: https://gerrit.libreoffice.org/71364
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>