Handle scaling factors

Currently squeekboard assumes a default Librem 5 screen at 360x720. When scaling the 720x1440 screen to higher DPI settings, the keyboard stays at the same 360x720 scale. That is to say, it doesn't grow to fill the same % of the screen, when the screen resolution increases. Here is an example of what I mean at 1x scale:

Ideally regardless of the screen resolution, squeekboard would take up the same % of space on the screen.