Using private apis in nuget packages

Since we don't provide any guarantees about private APIs being stable whatsoever, you need to pin the exact version of Avalonia in your package dependencies, like this: `

To proceed with private API usage, add <Avalonia_I_Want_To_Use_Private_Apis_In_Nuget_Package_And_Promise_To_Pin_The_Exact_Avalonia_Version_In_Package_Dependency>true</Avalonia_I_Want_To_Use_Private_Apis_In_Nuget_Package_And_Promise_To_Pin_The_Exact_Avalonia_Version_In_Package_Dependency> to your project file