Make the color of the page selection frame configurable

[TobiasKnauss]

Is your feature request related to a problem? Please describe. The only actual problem is, that the frame is sometimes hard to see. Please make the color configurable or choose a different color to get a higher contrast:

I have changed the options to save the selected pages instead of all pages when clicking the PDF button. This works nice; however I accidentally had a page selected and noticed later during double-check of the exported PDF.

Describe the solution you'd like The frame color should be configurable. And if possible, the frame line width, too.

Describe alternatives you've considered Choose a different default color.