stylecontext: Deprecate gtk_style_context_get_font()

stylecontext: Deprecate gtk_style_context_get_font()

This is for a very simple reason: The getter is returning a const value
and the font isn't const anymore. So we need to store the font
description somewhere but we can't reuse it as it's changing all the
time (yay animations, yay inherited values). Sucks.

So keep the hack in here but deprecate the function.