Make go back and forward buttons flat

[Camilo Higuita]

Right now the elementary theme doesn't theme correctly the buttons in 3.12+ and so you might not notice that some buttons are meant to be flat so they look nice. Scratch is a good example of how to style the headerbar buttons, while noise and pantheon-files don't use the flat buttons as they used to in Luna. I'll attach a screenshot:

[Danielle Foré (danrabbit)]

It looks like we have the "image-button" class, but not "flat". Doesn't effect our stylesheet, but would be good for others.

[Danielle Foré (danrabbit)]

For this one, we want to assign https://valadoc.org/gtk+-3.0/Gtk.STYLE_CLASS_FLAT.html

We do this by getting the style context of the widget and adding a style class like so:

widget.get_style_context ().add_class (foo);

where foo is your class name and in this case we want to use the constant Gtk.STYLE_CLASS_FLAT