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.
At the toplevel we have _gtk_theming_engine_get, which lets us pass
in a property context with (atm) the size to get the property for.
Then there is a lot of plumbing to push this down into the lower
layers of the style property code until finally hitting
the property resolvers.
I need this because I will be adding a property resolver for win32
theme parts, and they render differently depending on the size
(i.e. they don't scale linearly). The idea is that the code
to get the background properties will pass in the final size
and we will resolve the theme part specification to that particular
size.
If the old non-context calls are used we just hardcode a size
of 100x100.
