add a simple image and pdf import ability

BUG: 482224

Refs #13

Adds the possibility to import an image or a PDF and adds it to the document. Initially I envisioned a separate preview interface where the user can see and review the file to be imported again and apply the image size and DPI settings for the import for a perfect experience.

However, I figured that this solution will also suffice for the simpler use-cases and also allows to split up the code merge a bit. There is one caveat with this approach: We do not have the information anymore about the DPI with which an image was scanned with or what is supposed to be when it originally comes from a different source. As a workaround, the current DPI from the scanner is assumed to be the DPI of choice. However, this is nothing more than a heuristic and will not work for some cases.

I'm curious what happens when someone tries to import a 1000 page PDF...