Update Visual Studio projects and packaging.

* Everything up to date with latest changes, versioned to 0.5.0
* Updated [NuGet package](https://www.nuget.org/packages/libsodium_vc120) deployed
* Added VS project file filtering to match file system layout
* Refactored generic VS .props files (and related project integration) to reduce customization across project files for various VS versions, add flexibility and simplify naming conventions
* A couple of minor changes to explicitly use Unicode Win32 APIs
* Changes to output file locations/names are breaking for any projects taking a direct dependency. This is avoidable by instead importing the `libsodium.import.props` file into dependent projects or referencing the NuGet package.