Implement IPluginFactory3::setHostContext()

Now the plugin factories are fully implemented (at least, functionality
wise, we still can't create most kinds of objects).

README.md

@@ -17,13 +17,11 @@ imcomplete list of things that still have to be done before this can be used:
 
 - Left to implement:
   - `YaHostApplicationHostImpl::createComponent`.
-  - `IPluginFactory3::setHostContext()` using the same host application context proxy method.
   - The rest of `IComponent`'s functions
-  - `IPluginFactory3::setHostContext()`
   - All other mandatory interfaces
   - All other optional interfaces
 - Fully implemented:
-  - Nothing yet
+  - `GetPluginFactory()` and `IPluginFactory{,2,3}`
 - Update the GitHub Actions workflows.
 - Update yabridgectl to handle buth VST2 and VST3 plugins.
 - Update all documentation to refer to VST2 and VST3 support separately, and