Store context menu items in unordered maps

The difference in performance won't be noticable, but both lookups and
modifications in these things are much faster once you have more than
one or two elements.

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

@@ -357,7 +357,7 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
          * lot of details o how this should be implemented and there's no
          * example implementation around.
          */
-        std::map<int32, Steinberg::IPtr<YaContextMenuTarget>> targets;
+        std::unordered_map<int32, Steinberg::IPtr<YaContextMenuTarget>> targets;
     };
 
     /**