Move to AssemblyLoadContext

[rabbitism]

Currently there are issues loading Avalonia.Controls assembly with a different version from the version I'm using in this visualizer. It is necessary to utilize AssemblyLoadContext.

Changes include:

1. A collectible AssemblyLoadContext in MainWindowViewmodel
2. Report errors when a certain dependency is not available. This requires Semi.Avalonia to implement Notification first.
3. No longer refresh Assemblies upon dragging in, instead it will append the assembly. Provide feasibility to refresh and clear assemblies.
4. Pass a context around AssemblyViewModel and TypeViewModel. It is important to get the PropertyInfo of PseudoClassesAttribute first, as casting to this type is not a valid operation due to version conflict. It must be cast to the particular verison in this context.