Add configuration for backend sections

[discordier]

As commented in MetaModels/boilerplate#1 (comment), we want some configuration options to add custom sections in the backend.

be_sections:
    products:
        name:
            de: 'Produkte'
            en: 'Products'
        tooltip:
            de: 'Produkte'
            en: 'Products'
        icon: 'file/theme/mm/products.svg'
        add:
            before: layout

We might need more options here (like initial collapsed etc).