gsk: Avoid using gtk css types in public api

gsk: Avoid using gtk css types in public api

Using GtkCssSection in public headers here may be
ok from the C perspective, since it all ends up in
the same library anyway. But it causes circular
dependency problems for our gir files that are still
split by namespace.

To avoid this problem, copy the GtkCssLocation struct
struct as GskParseLocation, and pass take two of them
instead of a GtkCssSection in the error callback.

Update all users.

Fixes: #2454