Conan Mac Qt PCH fix

Lars Bilkerequested to merge
github/fork/bilke/mac-qt-fix into master
  • Fixed Conan QtXml package linking with PCH on macOS
  • added conditional Conan install option -o gui=True -> CLI devs do not need to download large Qt anymore, added instructions to the docs
  • Added a README.txt to the redistributable packages with usage instructions

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