Remove non-Toolchain content

[MarkSchofield]

This PR makes WindowsToolchain strictly just a CMake Toolchain, based on discussion #65. This reduces the surface area of the project, and as a result addresses #50 and most of #18 (moving and renaming files is still a discussion to have). I won't commit this until I've figured out where the other pieces land, but it seems useful to send this out to understand what a pure Toolchain would be.

This also removes the support for downloading VSWhere.exe if it can't be found. The Official documentation for finding Visual Studio instances enumerates the ways to find Visual Studio - including vswhere.exe - but never makes the statement that you can rely on the installation location of vswhere.exe. As a result, I added support for downloading vswhere.exe from an official distribution mechanism. I have since discovered (/noticed ;) that the VSWhere documentation does say that VSWhere should be found under %ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe, so I am going to rely on that.

[ClausKlein]

There is a small cmake-lint issue:

bash-3.2$ find . -name CMakeLists.txt | xargs cmake-lint 
./example/CommandLineModule/CMakeLists.txt
==========================================

./example/CMakeLists.txt
========================

./example/SharedLibrary/CMakeLists.txt
======================================

./example/CommandLine/CMakeLists.txt
====================================

./example/WindowsApplication/CMakeLists.txt
===========================================
./example/WindowsApplication/CMakeLists.txt:14: [C0304] Final newline missing

Summary
=======
files scanned: 5
found lint:
  Convention: 1

bash-3.2$

[MarkSchofield]

Thanks for taking a look, @ClausKlein. It turns out that my script was only linting *.cmake, and not CMakeList.txt. I've fixed that (and the lint you found) in #68.