settings: Provide a default style cascade per scale

This guarantees we only create 1 extra style cascade in total for hidpi
and not one per style context.

Style cascades are now nested like this:

GtkSettings root cascade (scale == 1)
 |
 +-- GtkSettings per scale cascade (for any scale, no custom providers)
 |
 +-- GtkStyleContext custom cascade (for any scale, custom providers)

This requires a bunch of care when changing cascade-related properties
inside GtkStyleContext, so that it ends up with a properly setup
cascade, but I think I got those cases right.

The only thing we don't do yet is reverting to a GtkSettings cascade
when the last custom provider is removed from a custom cascade.

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