Add the pieces for a watchdog for dangling plugins

Once we register our plugins here, we should be able to gracefully shut
down any plugins whose remote process got killed.

src/wine-host/bridges/common.cpp

@@ -34,3 +34,7 @@ void HostBridge::handle_win32_events() {
         DispatchMessage(&msg);
     }
 }
+
+void HostBridge::shutdown_if_dangling() {
+    // TODO: Implement
+}