Allow PDF import to import without any pages
NOTE: Forward porter, this is a GUI addition and might need tweaking to be forward ported to Gtk4
When importing a PDF, the new multi-page functionality disabled the ability to import PDF data as a bunch of shapes, instead importing everything as pages.
To fix this regression we:
- Add a checkbox to import pages, default is active
- When active, we ask the svg-builder to create pages
- We still import every page selected, but with the as_pages flag set
- The SVG builder creates a document without pages, but with a document width the size of the pages imported.
Fixes #4380