UICommon: Add AutoUpdate module + placeholder Qt implementation

The AutoUpdate module is a generic update checker mechanism which can be
used by UI backends to trigger an auto-update check as well as the
actual update process.

Currently only configurable through .ini and the Qt implementation is
completely placeholder-y -- blocking the main thread on a network
request on startup, etc.

Source/Core/UICommon/AutoUpdate.h

@@ -0,0 +1,38 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <string>
+
+// This class defines all the logic for Dolphin auto-update checking. UI-specific elements have to
+// be defined in a backend specific subclass.
+class AutoUpdateChecker
+{
+public:
+  // Initiates a check for updates in the background. Calls the OnUpdateAvailable callback if an
+  // update is available, does "nothing" otherwise.
+  void CheckForUpdate();
+
+  struct NewVersionInformation
+  {
+    // Name (5.0-1234) and revision hash of the new version.
+    std::string new_shortrev;
+    std::string new_hash;
+
+    // The full changelog in HTML format.
+    std::string changelog_html;
+
+    // Internals, to be passed to the updater binary.
+    std::string this_manifest_url;
+    std::string next_manifest_url;
+    std::string content_store_url;
+  };
+
+  // Starts the updater process, which will wait in the background until the current process exits.
+  void TriggerUpdate(const NewVersionInformation& info);
+
+protected:
+  virtual void OnUpdateAvailable(const NewVersionInformation& info) = 0;
+};