JUCE cpp files without header-file "#include" struggling with Intellisense

Because the most *.cpp files in JUCE don’t have any header-includes --> IntelliSense marks all objects as unknown.

Wouldn’t it be nicer add the required header-includes to each of the files?

That’s not possible - the cpps that I guess you’re talking about are not intended to be used as stand-alone compile units, they only work when included inside another file.