stylecontext: Deprecate functions

stylecontext: Deprecate functions

- gtk_style_context_get_background_color()
- gtk_style_context_get_border_color()

Those functions shouldn't be used anymore, because they don't represent
anything from the CSS styling we support. The background color often
isn't used due to background images and there are actually 4 different
border colors (1 for each side) - if there isn't also a border image in
use.