Feat/preparing for new base structure

This is not the base structure yet – I just pushed part of it because these are simple but breaking changes to the App component and regarding that we should be on the same page as soon as possible.

I just created a WindowManager component which is (for now) exactly the same as the App component was. This is just for more concise naming and to keep the App component clean and easy to extend.

But this shouldn’t affect any of your branches as long as you didn’t make changes to the App component or renamed the existing components. If you do any of those things in the future, please rebase first or just let me know.

Reviewer's task:

[x] I didn't see no code smells
[x] The live preview works as expected -> no regressions found

ISH-Gruppe / screenario

Feat/preparing for new base structure #20