Add an IConnectionPoint proxy implementation

We still have to pass this proxy to the plugin. That's next.

src/plugin/bridges/vst3-impls/plugin-proxy.h

@@ -124,6 +124,22 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
     tresult PLUGIN_API initialize(FUnknown* context) override;
     tresult PLUGIN_API terminate() override;
 
+    /**
+     * The component handler the host passed to us during
+     * `IEditController::setComponentHandler()`. When the plugin makes a
+     * callback on a component handler proxy object, we'll pass the call through
+     * to this object.
+     */
+    Steinberg::IPtr<Steinberg::Vst::IComponentHandler> component_handler;
+
+    /**
+     * If the host doesn't connect two objects directly in
+     * `IConnectionPoint::connect` but instead connects them through a proxy,
+     * we'll store that proxy here. This way we can then route messages sent by
+     * the plugin through this proxy. So far this is only needed for Ardour.
+     */
+    Steinberg::IPtr<Steinberg::Vst::IConnectionPoint> connection_point_proxy;
+
     /**
      * An unmanaged, raw pointer to the `IPlugView` instance returned in our
      * implementation of `IEditController::createView()`. We need this to handle
@@ -136,14 +152,6 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
      */
     Vst3PlugViewProxyImpl* last_created_plug_view = nullptr;
 
-    /**
-     * The component handler the host passed to us during
-     * `IEditController::setComponentHandler()`. When the plugin makes a
-     * callback on a component handler proxy object, we'll pass the call through
-     * to this object.
-     */
-    Steinberg::IPtr<Steinberg::Vst::IComponentHandler> component_handler;
-
     // The following pointers are cast from `host_context` if `setHostContext()`
     // has been called