fix issues found with cppclean

[pamapa]

https://github.com/myint/cppclean

cppclean can find the following:

• Classes with virtual methods, no virtual destructor, and no bases
• Global/static data that are potential problems when using threads
• Functions that are declared but not defined
• Unnecessary forward class declarations
• Unnecessary function declarations
• Undeclared function definitions
• Unnecessary #includes in header files
  • No direct reference to anything in the header
  • Header is unnecessary if classes were forward declared instead
• Inconsistent case in #includes (foo.h versus Foo.h)

[pamapa]

i plan to cleanup header handling like this:

example.cc:

/*
...
*/

#include "example.h" // API

#include <system headers_last>
...

#include "other_project_headers.h"
...

Like this our header files are self contained and do not depend included in a specific order.

[whyman]

Yay for the include tidy!

[whyman]

Once it's done consider a GitHub action to make sure it stays that way.

[pamapa]

cppclean shows many false positive, like this we can not add it to CI :-(