Draft: Add "resolution" argument to file-pdf-load API
This adds an oft-requested resolution argument to the file-pdf-load API12, and is thus related to #4807 (closed).
As resolution is already part of the underlying API, the only necessary change is to read the argument and overwrite the default.
I set a maximum of G_MAXUINT16 or 65535 as I could not find a better maximum constant relevant, and the actual max I got from the dialog was 1048576.