core: Add Exit and ExitCallback

This allows ISelfController::Exit to stop the currently running application. This is typically used by homebrew to exit back to the homebrew menu after calling consoleExit with libnx.

src/core/core.cpp

@@ -421,6 +421,7 @@ struct System::Impl {
     bool is_async_gpu{};
 
     ExecuteProgramCallback execute_program_callback;
+    ExitCallback exit_callback;
 
     std::array<u64, Core::Hardware::NUM_CPU_CORES> dynarmic_ticks{};
     std::array<MicroProfileToken, Core::Hardware::NUM_CPU_CORES> microprofile_dynarmic{};
@@ -798,6 +799,18 @@ void System::ExecuteProgram(std::size_t program_index) {
     }
 }
 
+void System::RegisterExitCallback(ExitCallback&& callback) {
+    impl->exit_callback = std::move(callback);
+}
+
+void System::Exit() {
+    if (impl->exit_callback) {
+        impl->exit_callback();
+    } else {
+        LOG_CRITICAL(Core, "exit_callback must be initialized by the frontend");
+    }
+}
+
 void System::ApplySettings() {
     if (IsPoweredOn()) {
         Renderer().RefreshBaseSettings();