There was not much missing to make LibreOffice export valid PDF/A-2,
so let's add that, and switch the UI to use that instead of A-1.

The old PDF/A-1 feature is still accessible via UNO / filter
parameter 'SelectPdfVersion':

- 1 gives you PDF/A-1a
- 2 gives you PDF/A-2b
- 16 gives you PDF 1.6

Change-Id: Iea4262b119bcf33b75f3d1406cc793bdcaec65d1
Reviewed-on: https://gerrit.libreoffice.org/69294
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>