Updates NuGet package of GLFW

Hi, I updated the NuGet package for GLFW once again.

The content is basically identical to before, but there are three important changes:

• The version number now uses four numbers: 3.2.1.5. The fourth number is a package-build number which I can increase, even though I pack the same version of GLFW. This finally allows me to get rid of the “pre-release” flags I previously used for the VS 2017 repackage.
• I no longer use CoApp. CoApp is dead and not really missed. At work I started writing .nuspec and .targets files manually, and this is neither very hard, nor very much work.
• Last but not least, I added some properties, available within the Visual Studio GUI as part of the properties pages. There you can now enforce configuration and toolset version of the library you want to use. This is especially useful if you have special configurations which cannot automatically mapped by NuGet. You can now map those manually, at least.

Of course, everything is available on bitbucket.

Now, I am waiting for the next version of GLFW to be published.