Windows binaries: Solve dependency to VS 2017 runtime

Current windows binaries depend on the C++ runtime from VS 2015 or 2017:

vcruntime140.dll
msvcp140.dll

While these files are often installed by other programs, they are not necessarily available on all machines.

There are multiple ways how the C++ runtime should be deployed (see also https://blogs.msdn.microsoft.com/vcblog/2015/03/03/introducing-the-universal-crt/ ):

1. Via a vcredist installer. Recommended by Microsoft, but requires Administrative privileges.
2. Statically linked into executable(s). Requires a custom build of Qt that is configured with static-runtime, and makes the executables bigger.
3. By adding all .dll's to the application directory (bin). This is not recommended by Microsoft, but avoids a separate installer / does not require Administrative privileges.

Qbs should arguably either add a vcredist installer to the Windows binary packages, or include the .dll's directly.