Reduce image resolution by default in PDF Export

PDF Files can quickly become very big when a user inserts some images. Most users don't know how to reduce the size of their images.
This patch enables the resolution reduction for PDF Export to 300DPI by default.

Change-Id: I3ce5191de35561b59169e52e2e2efbc4a343f025
Reviewed-on: https://gerrit.libreoffice.org/8422
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>