menbutton: Redo indicator arrows
Instead of hardcoding icon names in the widget, use arrow.none, arrow.up, arrow.down, arrow.left, arrow.right styles and set the icon to use with -gtk-icon-source. This lets themes change the icons that are used here, without forcing all uses of pan-up/down/start/end-symbolic to be treated the same. Document this in the menubutton CSS docs.
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