Prevent AnyCPU builds on .NET Framework

[Davipb]

As discussed in SFML/SFML.Net#174, the Nuget CSFML package doesn't copy its native files correctly when building with AnyCPU on the .NET Framework.

Given that the .NET Framework is being deprecated soon, this PR adds an MSBuild file to the package that detects when AnyCPU is being used and instructs the user to either switch to .NET Core or change platforms to x86/x64, failing the build.

A property is provided to override this error in case the user wants to copy and load CSFML's files manually at runtime.

Additionally, the deprecated PackageIconUrl property was replaced with PackageIcon pointing to a local PNG.

This should fix #149 and SFML/SFML.Net#174

[eXpl0it3r]

I'm running into the issue now even for .NET Core (.NET 5 / .NET Standard 2.0) that the error is triggered.
I also don't see any filtering for .NET framework, is that something you forgot in the end or some target magic where that shouldn't be called for .NET Core?

[Davipb]

The .props files are inside of the net folder, which used to be the Target Framework Moniker for the .NET Framework (while netcoreapp was the TFM for .NET Core), meaning that they would only be loaded while building for the .NET Framework.

But now with the rebranding of .NET Core to just .NET going forward, it seems like they decided to also rename the TFM for .NET Core to just net, meaning it now refers to both the .NET Framework and .NET 5+. This is a really baffling move that I just discovered while researching the cause of this issue and is probably going to break a lot of stuff.

It seems like you can target just the .NET Framework now by using the netframework TFM, but I need to do some experimenting to confirm this. I'll reply back when I know more.

[Davipb]

I can confirm using netframework now works as expected: Errors out on the .NET Framework but not on .NET 5.

[eXpl0it3r]

Thanks for looking into it. Do you want to send a PR or should I write it?

[Davipb]

Fastest PR in the west 🤠