Move Win32 event handling to one place

Or technically, two, since the group bridge also does the same loop. We
no longer need special handling for VST2 and VST3 plugins, so we can
simplify things a bit here.

src/wine-host/bridges/common.h

@@ -80,11 +80,8 @@ class HostBridge {
      * specific situation that can cause a race condition in some plugins
      * because of incorrect assumptions made by the plugin. See the dostring for
      * `Vst2Bridge::editor` for more information.
-     *
-     * TODO: We can get rid of this now, since we no longer have any special
-     *       handling here
      */
-    virtual void handle_win32_events() = 0;
+    void handle_win32_events();
 
     /**
      * The path to the .dll being loaded in the Wine plugin host.