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/UICommon.cpp

@@ -2,16 +2,20 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
+#include <cmath>
 #include <memory>
 #ifdef _WIN32
 #include <shlobj.h>  // for SHGetFolderPath
 #endif
 
+#include "Common/Common.h"
 #include "Common/CommonPaths.h"
 #include "Common/Config/Config.h"
 #include "Common/FileUtil.h"
 #include "Common/Logging/LogManager.h"
+#include "Common/MathUtil.h"
 #include "Common/MsgHandler.h"
+#include "Common/StringUtil.h"
 
 #include "Core/ConfigLoaders/BaseConfigLoader.h"
 #include "Core/ConfigManager.h"
@@ -313,4 +317,21 @@ void EnableScreenSaver(bool enable)
 #endif
 }
 
+std::string FormatSize(u64 bytes)
+{
+  // i18n: The symbol for the unit "bytes"
+  const char* const unit_symbols[] = {_trans("B"),   _trans("KiB"), _trans("MiB"), _trans("GiB"),
+                                      _trans("TiB"), _trans("PiB"), _trans("EiB")};
+
+  // Find largest power of 2 less than size.
+  // div 10 to get largest named unit less than size
+  // 10 == log2(1024) (number of B in a KiB, KiB in a MiB, etc)
+  // Max value is 63 / 10 = 6
+  const int unit = IntLog2(std::max<u64>(bytes, 1)) / 10;
+
+  // Don't need exact values, only 5 most significant digits
+  const double unit_size = std::pow(2, unit * 10);
+  return StringFromFormat("%.2f %s", bytes / unit_size, GetStringT(unit_symbols[unit]).c_str());
+}
+
 }  // namespace UICommon