Unify ISOFile (wx) with GameFile (Qt) and put it in UICommon

The original reason I wanted to do this was so that we can replace
the Android-specific code with this in the future, but of course,
just deduplicating between DolphinWX and DolphinQt2 is nice too.

Fixes:

- DolphinQt2 showing the wrong size for split WBFS disc images.

- DolphinQt2 being case sensitive when checking if a file is a DOL/ELF.

- DolphinQt2 not detecting when a Wii banner has become available
after the game list cache was created.

Removes:

- DolphinWX's ability to load PNGs as custom banners. But it was
already rather broken (see https://bugs.dolphin-emu.org/issues/10365
and https://bugs.dolphin-emu.org/issues/10366). The reason I removed
this was because PNG decoding relied on wx code and we don't have any
good non-wx/Qt code for loading PNG files right now (let's not use
SOIL), but we should be able to use libpng directly to implement PNG
loading in the future.

- DolphinQt2's ability to ignore a cached game if the last modified
time differs. We currently don't have a non-wx/Qt way to get the time.

Source/Core/UICommon/GameFileCache.h

@@ -0,0 +1,58 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <cstddef>
+#include <functional>
+#include <list>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <vector>
+
+#include "Common/CommonTypes.h"
+
+class PointerWrap;
+
+namespace Core
+{
+class TitleDatabase;
+}
+
+namespace UICommon
+{
+class GameFile;
+
+std::vector<std::string> FindAllGamePaths(const std::vector<std::string>& directories_to_scan,
+                                          bool recursive_scan);
+
+class GameFileCache
+{
+public:
+  void ForEach(std::function<void(const std::shared_ptr<const GameFile>&)> f) const;
+
+  void Clear();
+
+  std::shared_ptr<const GameFile> AddOrGet(const std::string& path, bool* cache_changed,
+                                           const Core::TitleDatabase& title_database);
+
+  // These functions return true if the call modified the cache.
+  bool Update(const std::vector<std::string>& all_game_paths);
+  bool UpdateAdditionalMetadata(const Core::TitleDatabase& title_database);
+
+  bool Load();
+  bool Save();
+
+private:
+  bool UpdateAdditionalMetadata(std::shared_ptr<GameFile>* game_file,
+                                const Core::TitleDatabase& title_database);
+
+  bool SyncCacheFile(bool save);
+  void DoState(PointerWrap* p, u64 size = 0);
+
+  std::vector<std::shared_ptr<GameFile>> m_cached_files;
+};
+
+}  // namespace UICommon