Fix: Defining two icons by element #587
Description
By the changes out of #509 it's currently impossible to define not just one (before or after), but two icons (before and after) on an element.
That for e.g. the main navigation is missing its initial "burger" icon, whereas the close icon is incorrectly displayed on the left side: https://db-ui.github.io/core/?p=pages-sample-page