Apply input config changes on server start

Initially, the config file is loaded before the server starts. The
changes need some server components to be applied, so they aren't
applied at that stage.

The few lines added store the changes and apply them once the server
starts.

examples/mir_demo_server/server_example.cpp

@@ -121,15 +121,15 @@ try
     miral::MirRunner runner{argc, argv, "mir/mir_demo_server.config"};
 
     miral::InputConfiguration input_configuration;
+    auto mouse = input_configuration.mouse();
+    auto touchpad = input_configuration.touchpad();
+    auto keyboard = input_configuration.keyboard();
 
     miral::ConfigFile test{runner, "mir_demo_server.input", miral::ConfigFile::Mode::reload_on_change,
-        [&input_configuration](auto& in, auto path)
+        [&](auto& in, auto path)
     {
         std::cout << "** Reloading: " << path << std::endl;
 
-        auto mouse = input_configuration.mouse();
-        auto touchpad = input_configuration.touchpad();
-        auto keyboard = input_configuration.keyboard();
 
         for (std::string line; std::getline(in, line);)
         {
@@ -190,6 +190,14 @@ try
         input_configuration.keyboard(keyboard);
     }};
 
+    runner.add_start_callback(
+        [&]
+        {
+            input_configuration.mouse(mouse);
+            input_configuration.touchpad(touchpad);
+            input_configuration.keyboard(keyboard);
+        });
+
     runner.set_exception_handler(exception_handler);
 
     std::function<void()> shutdown_hook{[]{}};