Allow overriding OS zoom level

[ecton]

When making a game, you might want to allow the user to select a custom UI scale. This should be applied across the entire window.

When embedding Cushy, the scale factor is just something that can be set. I see two ways of doing this:

• Allow setting the scaling factor through a Dynamic. Once it has been set through the dynamic, changes in the DPI scaling factor will not be applied. I don't love that this approach prevents DPI scaling from being applied.
• Add another setting to the window: Zoom level. This value would be multiplied against the scaling factor to produce the actual zoom factor of the window.

If we do both approaches, in theory any logic can be written: full override or dpi-aware zoom levels.