There have been quite a bit of number of questions how to change scale of the gui, because this option is hidden from the user.
This is exposing it using bounds from 0.1 to 10 and requires application restart.
0.1 instead 0 because scale of 0 is dumm but 0.5 is reasonable so maybe 0.1 for someone is reasonable too.
10 looks like overshooting to me and I do not think someone would need to scale gui above 10.
Q: should be max number be removed, or is there some another lower reasonable max bound (5?, 4?) ?
Q: slider vs number - I see disadvantage of slider with decimal numbers if user wants some exact number, I think it is easier to type it
Q: it is gui, so is general tab acceptable or we need to clean/split graphic options
!Q: There is still possibility that user will overshoot scaling and will have to edit config file to get application to usable state.