Port all samples to .NET Core / 4.7.2

The existing `Samples` folder is mostly copied from the original FreeImage .NET Wrapper.

* Delete the VS2005 `.csproj` files
* Replace the existing `.csproj` files with new SDK-style projects  multi-targeting `netcoreapp2.1` and `net472`
* The samples that target WinForms will need to be `net472` only for now. These can have `netcoreapp3.0` added as a target in future