Add support for unloading Assembly with WPF control in .NET Core

The .NET Core 3.0 has a new feature to enable unloading assemblies from memory, called unloadable AssemblyLoadContext. This feature is useful e.g. for implementing unloadable plugins.

In WPF, the `MS.Internal.Resources.ResourceManagerWrapper` holds on the assemblies containing controls forever (the assemblies are added to an internal container in the `AppDomain.CurrentDomain.AssemblyLoad` event handler and never removed).
So when the assembly is loaded into an unloadable `AssemblyLoadContext`, the resource container prevents the assembly from ever being unloaded, as the `AssemblyLoadContext` unloadability relies on GC and all references to the assembly need to go away before it can be unloaded.

To make WPF unloadability friendly, adding an ability to remove an assembly from the resource container would be needed.

It seems the following changes should be sufficient:
* In the existing `AppDomain.CurrentDomain.AssemblyLoad` event handler in the `ResourceContainer` , the resource manager hooks the `AssemblyLoadContext.Unloading` event for all assemblies that have `IsCollectible` property true. The `AssemblyLoadEvent.GetLoadContext(Assembly)` would be used to get the context to hook the event handler on from the assembly.
* When the `Unloading` event fires, the resource container removes the assembly from its cache