Introducing cppclean scaning

[atobiszei]

• Add script for veryfing count of warnings from cppclean
• Add additional step in sdl-check to check cppclean as well
• Remove unused forward declares, mark local functions as static
• Introduce forward declares instead of includes when possible

Script has hardcoded value of warnings. This is to be lowered with each incoming fix. Several issues were fixed already.

There seems to be issue with false-positives when using c++ "using" directive in separate header files. In this case we have to live with the issue reported by cppclean.