Add default-decoration style class constant

[cassidyjames]

I thought this might be a GTK thing but it looks like it's not defined by them or us. Since we use it and encourage devs to use it, we should define it as a constant.

[danirabbit]

Yeah Gtk uses it internally, but they don’t have a constant for it.

I kind of feel like it might be better to create a more semantic class name in case Gtk changes this upstream and it breaks all our styles.

[cassidyjames]

@danrabbit I don't feel that strongly, but how would it break for apps that are adding the style class via the constant? The nice thing about encouraging using a constant is that we can change it under the hood later if we want to track GTK, or just change it to something else later.