Allow entering % values on the ScreenRes.txt files

**Is your feature request related to a problem? Please describe.**
ScreenRes file entry can be cumbersome and always differ between installation

**Describe the solution you'd like**
If the backglass is enterered as width 100% it means 100% of the selected screen width
If the backglass is enterered as height 80% it means 80% of the selected screen heigth
