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.h

@@ -5,10 +5,14 @@
 #pragma once
 
 #include <string>
+#include <vector>
+
 #include <wx/colour.h>
 #include <wx/gdicmn.h>
 #include <wx/string.h>
 
+#include "Common/CommonTypes.h"
+
 class wxControl;
 class wxBitmap;
 class wxImage;
@@ -18,6 +22,11 @@ class wxToolBar;
 class wxTopLevelWindow;
 class wxWindow;
 
+namespace UICommon
+{
+struct GameBanner;
+}
+
 namespace WxUtils
 {
 // Launch a file according to its mime type
@@ -95,6 +104,9 @@ constexpr LSIFlags operator&(LSIFlags left, LSIFlags right)
   return static_cast<LSIFlags>(static_cast<unsigned int>(left) & right);
 }
 
+wxImage ToWxImage(const UICommon::GameBanner& banner);
+wxImage ToWxImage(const std::vector<u32>& buffer, int width, int height);
+
 // Swiss army knife loader function for preparing a scaled resource image file.
 // Only the path and context are mandatory, other parameters can be ignored.
 // NOTE: All size parameters are in window pixels, not DIPs or framebuffer pixels.