All code now lives either in PCL or Shared projects. All
platform-specific projects have been placed in a Platforms
solution folder, since all they do is reference PCLs and
Shared projects.
The platform projects must be named the same for the PCL
bait&switch behavior to kick in.
Simplified how shared .props y .targets are used/imported,
moved .snk to a single place, improved internals visible to,
etc.
Daniel Cazzulino