C++11 iwyu
So far, the include-what-you-use
checks have only been performed in C++98 mode.
Enabling the checks in C++14 mode shows additional warnings.
This MR fixes most of the warnings and disables the tool on cmake-gui. Further additions to the mapping are necessary for checking cmake-gui.
I was not able to fix all warnings. See this issue in iwyu: https://github.com/include-what-you-use/include-what-you-use/issues/434