Modernize Codebase to C++17

[nschimme]

We currently can't upgrade to C++17 because of a few factors:

• Supported C++17 compilers are: https://en.cppreference.com/w/cpp/compiler_support

  • GCC 7
  • MSVC 19.04
  • Clang 5

• This means we can only support the following OS / compiler combinations:

  • Ubuntu 18.04 (GCC 7)
  • Windows MSVC (19.14)
  • Mac Xcode 9 (Clang 5)

• Without support for:

  • Ubuntu 16.04 (GCC 5)
  • Windows MinGW (GCC 4.7.2)
  • Windows Mingw-w64 (GCC 6)

[nschimme]

Additionally, snaps are currently built on core16 and are blocked from migrating to core18 until https://bugs.launchpad.net/ubuntu/+source/docker.io/+bug/1755250 is fixed

Edit: I have succesfully built the snap on core18. This also means that we are able to support 16.04 users via snaps.

[nschimme]

It seems that we have another blocker. Mac only supports std::optional on macOS 10.14 and higher but we still have a substantial portion of our users on 10.11.

~/MMapper/src/expandoracommon/room.cpp:116:24: error:
      call to unavailable member function 'value': introduced in macOS 10.14
            return opt.value();
                   ~~~~^~~~~

[nschimme]

This has been completed now.