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/DolphinWX/WxUtils.cpp

@@ -28,6 +28,8 @@
 
 #include "DolphinWX/WxUtils.h"
 
+#include "UICommon/GameFile.h"
+
 #ifdef _WIN32
 #include <Windows.h>
 #endif
@@ -285,6 +287,27 @@ wxSize GetTextWidgetMinSize(const wxSpinCtrl* spinner)
   return size;
 }
 
+wxImage ToWxImage(const UICommon::GameBanner& banner)
+{
+  return ToWxImage(banner.buffer, banner.width, banner.height);
+}
+
+wxImage ToWxImage(const std::vector<u32>& buffer, int width, int height)
+{
+  wxImage image(width, height, false);
+  if (buffer.empty())
+    return image;
+
+  unsigned char* data = image.GetData();
+  for (int i = 0; i < width * height; i++)
+  {
+    data[i * 3 + 0] = (buffer[i] & 0xFF0000) >> 16;
+    data[i * 3 + 1] = (buffer[i] & 0x00FF00) >> 8;
+    data[i * 3 + 2] = (buffer[i] & 0x0000FF) >> 0;
+  }
+  return image;
+}
+
 static wxImage LoadScaledImage(const std::string& file_path, const wxWindow* context,
                                const wxSize& output_size, const wxRect& usable_rect, LSIFlags flags,
                                const wxColour& fill_color)