Compile NVDA with Visual Studio 2017. (#7568)
* Switch to using SCons 3.0 from a git mirror we manage.
* Compile NVDA with the Windows 10 SDK, replace GetVersion with versionHelper macros, and remove a pre-vista TSF check.
* Instruct appveyor to use its Visual Studio 2017 image
* Update readme to mention Visual Studio 2017
* iSimpleDOMNode_sconscript: don't lose existing flags when adding c_ext and /I to MIDLFLAGS as this was dropping /x64 which is needed for compiling when 64 bit.
* nvdaHelper sconscripts: now we are always using the Windows 10 SDK, we can compile nvdaHelperLocalWin10 using the same infrastructure as nvdaHelperRemote and nvdaHelperLocal etc, except that rather than /mt, /ZW should be used. This removes a great deal of code from the nvdaHelperLocalWin10 sconscript.
* Readme.md: update VS 2017 section with extra needed dependency
* Clarify error message when VC++ 14.1 is missing.
* Ensure that all Python directories exist, eitherwise raise an error alerting the user to the fact they may need to run git submodule update --init
* nvdaHelperLocalWin10 sconscript: search versioned directories from newest to oldest to find the vc restributables.
