Add support for Clang-Tidy

[hugolm84]

This PR adds support for Clang-Tidy by simply passing -DCLANG_TIDY=ON when configuring CMake.
If you want to let Clang-Tidy try to fix the suggestions, warnings and errors Automagically™ just pass -DCLANG_TIDY_FIX=ON.

Build as usual using cmake --build . (or ninja, make etc)

I took the liberty to choose the clang-tidy checks at my own liking. Should any of them not be suitable for this project, please comment.

Tested on Linux, OSX and Windows.

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[gdamore]

I'll have a go at this -- I want to test it out for myself before I merge.

[gdamore]

Mostly this is on hold waiting for me to review and assess.
I'm using a clang-tidy integration with my editor now, which is helping, along with checks being done via LGTM during the PR process.

[gdamore]

I've not forgotten about this PR -- it's not a huge priority for me however. I will probably work towards getting something based on this work in later -- I really want to clean up the mess that is our CMake support first though.