Introduce an Event system to VideoCommon

A lot of the remaining complexity in Renderer is the massive Swap function
which tries to handle a bunch of FrameBegin/FrameEnd events.

Rather than create a new place for it. This event system will try
to distribute it all over the place

Source/Core/Common/EventHook.h

@@ -0,0 +1,79 @@
+// Copyright 2023 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "Common/Logging/Log.h"
+#include "Common/StringLiteral.h"
+
+#include <functional>
+#include <memory>
+#include <string>
+#include <string_view>
+#include <vector>
+
+// A hookable event system.
+
+// Define Events in a header as:
+//
+//     using MyLoveyEvent = Event<"My lovely event", std::string>;
+//
+// Register listeners anywhere you need them as:
+//    EventHook myHook = MyLoveyEvent::Register([](std::string foo) {
+//       // Do something
+//   }, "Name of the hook");
+//
+// The hook will be automatically unregistered when the EventHook object goes out of scope.
+// Trigger events by doing:
+//
+//   MyLoveyEvent::Trigger("Hello world");
+//
+
+struct HookBase
+{
+  virtual ~HookBase() = default;
+};
+
+using EventHook = std::unique_ptr<HookBase>;
+
+template<StringLiteral EventName, typename... CallbackArgs>
+class Event
+{
+public:
+  using CallbackType = std::function<void(CallbackArgs...)>;
+
+private:
+  struct HookImpl : public HookBase
+  {
+    ~HookImpl() override { Event::Remove(this); }
+    HookImpl(CallbackType callback, std::string name) : m_fn(callback), m_name(name){ }
+    CallbackType m_fn;
+    std::string m_name;
+  };
+public:
+
+  // Returns a handle that will unregister the listener when destroyed.
+  static EventHook Register(CallbackType callback, std::string name)
+  {
+    DEBUG_LOG_FMT(COMMON, "Registering {} handler at {} event hook", name, EventName.value);
+    auto handle = std::make_unique<HookImpl>(callback, name);
+    m_listeners.push_back(handle.get());
+    return handle;
+  }
+
+  static void Trigger(CallbackArgs... args)
+  {
+    for (auto& handle : m_listeners)
+      handle->m_fn(args...);
+  }
+
+private:
+  static void Remove(HookImpl* handle)
+  {
+    auto it = std::find(m_listeners.begin(), m_listeners.end(), handle);
+    if (it != m_listeners.end())
+      m_listeners.erase(it);
+  }
+
+  inline static std::vector<HookImpl*> m_listeners = {};
+};