This patch attempts to allow users to export PDFs with transparent backgrounds.
It follows the simpler, PDF-only suggestion offered by Thomas Weißschuh here. If the more general, plug-in option discussed in the issue page is preferred, this merge request will be revised accordingly.
Note: It might be good to offer this option on import as well. Currently all PDFs load with a white background automatically added, which would cause problems if you wanted to edit a transparent PDF. ImageMagick opens all PDFs as transparent by default, which is fine for single-page PDFs but I think causes issues with multi-page PDFs.
Proposed UI:
