Transition to C++17

[rakhimov]

System:

• [x] Compiler versions (gcc, clang, icc)
• [x] Compiler flags (cmake upgrade)
• [x] Travis-CI (linux, macos)
• [x] Appveyor-CI (windows mingw64)
• [x] Qt

Replace Boost:

• [ ] filesystem
• [x] variant
• [x] optional
• [x] string_ref

Docs:

• [x] Upgrade coding standards

[rakhimov]

clang 5.0 fails with libstdc++ std::variant: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=877838

[rakhimov]

std::optional and std::variant are missing from macOS Xcode 9: https://forums.developer.apple.com/thread/79555

[rakhimov]

<filesystem> is not yet available by default w/ GCC 7 or Clang 5. Hopefully, it will be available in GCC 8 and Clang 6.

[rakhimov]

Clang-format 5.0 gets confused with C++17 structured bindings: https://reviews.llvm.org/D37132

[rakhimov]

Qt5 moc doesn't understand C++17 nested namespace definition.