Toolbar overflow button: add a better looking arrow

[manuq]

See ​this comment on ticket #4192, which has a ​mockup and the icon attached.

[manuq]

Seems not possible in current GTK. The overflow button is a private GtkToggleButton that has a GtkArrow inside:

​http://git.gnome.org/browse/gtk+/tree/gtk/gtktoolbar.c#n689

The arrow can be styled, and we already did, because it has a name:

gtk_widget_set_name (priv->arrow, "gtk-toolbar-arrow");

In order to style the togglebutton, we should send a patch to gtk+. We could then set a background-image to it.

For now, at least we can improve a little bit making the arrow bigger:

.toolbar .button #gtk-toolbar-arrow {
    padding: 0 $(subcell_size + default_padding)px;
+    -GtkArrow-arrow-scaling: 2.0;
}