Use DefaultPlatformToolset for .vcxproj files

Use DefaultPlatformToolset for .vcxproj files

- Use $(DefaultPlatformToolset) for the PlatformToolset
  instead of v140. This allows to use the associated PlatformToolset
  automatically for each version of Visual Studio:
  v140 for VS2015, v141 for VS2017, v142 for VS2019.
  However, a developer can easily target projects to a desired,
  different PlatformToolset.
- Set ToolsVersion="Current" instead of ToolsVersion="14.0"
- Strings were replaced in .vcxproj files the following way:
  git grep -l 'v140' -- '*.vcxproj' | \
    xargs sed -i 's/v140/$(DefaultPlatformToolset)/g'
  git grep -l '14.0' -- '*.vcxproj' | xargs sed -i 's/14.0/Current/g'