Add GtkStylePropertyContext and use it in when getting style properties
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.
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