VS project gen: add fully redefined mappings

Visual Studio croaks if you try to reinvent a completely new platform, like
"Flub", and it will just build Win32 instead. This ends up happening with
cross-compilation builds that target some non-VS platform, say "Linux."

For cases like this you can now remap configuration names completely and
stick more information in the variant name instead, and let VS pretend
that you're still building Win32. Here's how:

  IdeGenerationHints = {
    Msvc = {
      PlatformMappings = {
        ['win32-msvc']    = 'Win32',
        ['linux-mingw']   = 'Win32',
      },
      FullMappings = {
        ['win32-msvc-debug-default']         = { Config='Debug',              Platform='Win32' },
        ['win32-msvc-production-default']    = { Config='Production',         Platform='Win32' },
        ['win32-msvc-release-default']       = { Config='Release',            Platform='Win32' },
        ['linux-mingw-debug-default']        = { Config='Debug Linux',        Platform='Win32' },
        ['linux-mingw-production-default']   = { Config='Production Linux',   Platform='Win32' },
        ['linux-mingw-release-default']      = { Config='Release Linux',      Platform='Win32' },
      },
    },

To build the linux config, simply select "Debug Linux" instead of "Debug" from the config dropdown in VS.