Fractional Scale under 100%

[cyrneko]

Problem

Sometimes there's a situation where one wants to make the UI smaller than 100%, like on a relatively large, but low-resolution screen (some laptops I've used had this)

Proposal

elementary/gala should allow setting the display scale to less than 100%, to shrink down the UI

Prior Art (Optional)

KDE Plasma supports this in their scaling options, by just setting the scale to a percentage value like 70% it will scale down the UI.