Add a function for temporarily blocking event loop

This can be used to prevent the Win32 message loop from running while
there are plugins in some partially initialized state.

src/wine-host/bridges/common.h

@@ -32,6 +32,20 @@ class HostBridge {
    public:
     virtual ~HostBridge(){};
 
+    /**
+     * If a plugin instance returns `true` here, then the event loop should not
+     * be run. Some very specific plugins, like the T-RackS 5 plugins, will set
+     * up a Win32 timer in their constructor, but since the plugins are left in
+     * a partially initialized state until `effOpen()` has been called running
+     * the Win32 message loop before that time will trigger a race condition
+     * within those plugins. This is very much an issue with those plugins, but
+     * since this situation wouldn't occur on Windows we'll just have to work
+     * around it.
+     *
+     * @relates MainContext::async_handle_events
+     */
+    virtual bool inhibits_event_loop() = 0;
+
     /**
      * Handle events until the plugin exits. The actual events are posted to
      * `main_context` to ensure that all operations to could potentially
@@ -66,6 +80,9 @@ 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;