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.h

@@ -65,7 +65,7 @@ class PluginBridge {
      * Ask the VST plugin to process audio for us. If the plugin somehow does
      * not support `processReplacing()` and only supports the old `process()`
      * function, then this will be handled implicitely in
-     * `WineBridge::handle_process_replacing()`.
+     * `Vst2Bridge::handle_process_replacing()`.
      */
     void process_replacing(AEffect* plugin,
                            float** inputs,