New design for fractional scaling setting

The current fractional scaling setting is just a long list of percentage options. If this setting is going to be exposed more widely (which I think it is), we ought to do a better job. Specifically, we need to reduce the number of options presented and ideally have some kind of preview.

The other thing we need is a switch for the legacy compatibility mode, to allow overriding the defaults, in order for people to use certain X11 apps.

The designs are part of the latest display settings mockups and look like this: