Fully implement IContextMenu

Although all of this stuff is completely untested, and since no host on
Linux uses it we'll likely never know whether this implementation is
correct.

src/wine-host/bridges/vst3-impls/context-menu-proxy.h

@@ -46,17 +46,23 @@ class Vst3ContextMenuProxyImpl : public Vst3ContextMenuProxy {
     addItem(const Steinberg::Vst::IContextMenuItem& item,
             Steinberg::Vst::IContextMenuTarget* target) override;
     tresult PLUGIN_API
-    removeItem(const Item& item,
+    removeItem(const Steinberg::Vst::IContextMenuItem& item,
                Steinberg::Vst::IContextMenuTarget* target) override;
     tresult PLUGIN_API popup(Steinberg::UCoord x, Steinberg::UCoord y) override;
 
     /**
      * The targets passed when to `addItem` calls made by the plugin. This way
-     * we can call these same targets later.
+     * we can call these same targets later. The key here is the item's tag.
      */
     std::map<int32, Steinberg::IPtr<Steinberg::Vst::IContextMenuTarget>>
         context_menu_targets;
 
    private:
     Vst3Bridge& bridge;
+
+    /**
+     * The items passed when to `addItem` calls made by the plugin. This way we
+     * can call these same targets later.
+     */
+    std::vector<Steinberg::Vst::IContextMenuItem> items;
 };