Move wine-bridge.h -> bridges/vst2.h

This way we can structure the group handling and a potential future VST3
bridge in the same way.

src/plugin/plugin-bridge.cpp

@@ -165,7 +165,7 @@ PluginBridge::PluginBridge(audioMasterCallback host_callback)
         try {
             while (true) {
                 // TODO: Think of a nicer way to structure this and the similar
-                //       handler in `WineBridge::handle_dispatch_midi_events`
+                //       handler in `Vst2Bridge::handle_dispatch_midi_events`
                 receive_event(
                     vst_host_callback, std::pair<Logger&, bool>(logger, false),
                     [&](Event& event) {