Remove _gtk_style_properties_peek_property

Remove _gtk_style_properties_peek_property

We want to move to a world where getting style properties can
allocate a new value, depending on the style property context.
In this world we can't expose "peeking" a property, as we neet
to return a ref to the property that is newly created.

So, we move the peek code into get_property and use get_property
from get_valist.