Compatibility for Contao > 5.x

[cliffparnitzky]

Due to the fact there is no install tool for Contao 5, we need a migration job. For example something like this: https://github.com/contao-themes-net/nature-theme-bundle/blob/master/src/Migration/InitialDemoDataMigration.php

For this purpose, the structure must also be changed to bundle structure.