Draft: Update document DPI when window screen changes.

This means that the document stays the correct size when moving the window between monitors, instead of simply sticking with the DPI of which monitor you opened the document on.

This is my first time working on any sort of Qt app and I have little C++ experience so bear with me here. I still need to fix document generators outside PDF but I'd like to at least make sure I'm not screwing everything up here.