Make `Widget.get_style_context() const` return a `StyleContext const`?

[Daniel Boles <dboles src gmail com>]

I suspect this might have been asked and discredited already, so if it has, apologies and link me :)

In gtk/src/widget.hg we have:

```

  _WRAP_METHOD(Glib::RefPtr<StyleContext> get_style_context(), gtk_widget_get_style_context, refreturn)
  _WRAP_METHOD(Glib::RefPtr<Gtk::StyleContext> get_style_context() const, gtk_widget_get_style_context, refreturn, constversion)

```

Should the const version return a RefPtr to a `const StyleContext`? I feel like perhaps it should.

If we have the style context, we can change how the widget is drawn, which is arguably quite a non-const thing to do with something whose main purpose is to be displayed :-)