Style arrows

[manuq]

Our arrows have regressed in the port to GTK+3. See comparison:

This can't be done just with CSS. This is a special case that needs a new engine
written in C, that reimplements gtk-render-arrow of GtkStyleContext. The new engine can inherit only this code and nothing else, so we are safe doing it.

Reference: ​http://developer.gnome.org/gtk3/3.0/GtkThemingEngine.html

After that we can style in the CSS with this rules:

 * {
     engine: sugar;
 }

.arrow {
    color: #f00;
}

 * {
-GtkArrow-arrow-size: 1.0;
-GtkWidget-scroll-arrow-hlength: 11;
-GtkWidget-scroll-arrow-vlength: 11;
-GtkMenu-scroll-arrow-vlength: 19;
}