Add styling support

[stackotter]

Currently SwiftGtk doesn't provide a Swift-y way to style widgets. To support styling, SwiftGtk needs to wrap Gtk's css-based styling API.

The main Gtk functions that would need to be wrapped are css_provider.load_from_data, gtk_style_context_add_class, gtk_widget_get_style_context. There may be others that I have missed.

[stackotter]

I've implemented a basic way to style elements which can be expanded on in future. Further development of SwiftGtk will be happening in the SwiftCrossUI repo.