Convert this project to TypeScript (if we do, asap)

[pklaschka]

Since the project appears to get relatively big, it might be useful to move to TypeScript, at least for the main project, as this would allow type checking etc., giving a lot more confidence etc...

Converting, at an early stage, wouldn't be a lot of work and I also could take this on (I actually got a new book, Sebastian Springer: React - Das umfassende Handbuch (Rheinwerk 2019) where a chapter explains how to convert a project to TypeScript).

Opinions?