With this commit a unified structure for includes is introduced.
The general rules:
* Only include what you need
* Include wherever possible in the cpp and forward-declare in the header
The includes are sorted in a local to global fashion. This means for the include order:
0. If cpp file: The corresponding h file and an empty line
1. Includes from the project sorted alphabetically
2. System/library includes sorted alphabetically
3. Conditional includes
