Run events async and centralized for group hosts

At a 30 fps rate with a limit on the number of window messages per
frame. This is somehow needed for Melda plugins, as they otherwise
dispatch tiemr messages indefinitely after opening a second editor with
seemingly no way around it.

With this and some refactoring #15 should be almost done.

src/wine-host/bridges/group.h

@@ -183,6 +183,13 @@ class GroupBridge {
      */
     void accept_requests();
 
+    /**
+     * Handle both Win32 messages and X11 events on a timer within the IO
+     * context. This is a centralized replacement for the event handling in
+     * `Vst2Bridge::handle_dispatch_single` for plugin groups.
+     */
+    void async_handle_events();
+
     /**
      * Continuously read from a pipe and write the output to the log file. Used
      * with the IO streams captured by `stdout_redirect` and `stderr_redirect`.
@@ -268,6 +275,14 @@ class GroupBridge {
      */
     std::mutex active_plugins_mutex;
 
+    /**
+     * A timer used to repeatedly handle the Win32 message loop and the X11
+     * events.
+     *
+     8 @see async_handle_events
+     */
+    boost::asio::steady_timer events_timer;
+
     /**
      * A timer to defer shutting down the process, allowing for fast plugin
      * scanning without having to start a new group host process for each