GtkStyleContext deprecation warning

[phastmike]

GtkStyleContext is going to be deprecated but the static methods add/remove_provider_for_display aren't but the vala compiler does warn about deprecation. It might be a small mistake or side effect of deprecating the class and as a consequence we get the warning but nevertheless it shouldn't.

Added this info here just for future reference:

../src/preferences-window.vala:90.13-90.28: warning: `Gtk.StyleContext' has been deprecated since 4.10
   90 |             Gtk.StyleContext.add_provider_for_display (Gdk.Display.get_default (), provider, Gtk.STYLE_PROVIDER_PRIORITY_USER);

[phastmike]