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/DolphinQt2/MainWindow.h

@@ -11,6 +11,7 @@
 
 #include <memory>
 #include <optional>
+#include <string>
 
 #include "DolphinQt2/GameList/GameList.h"
 #include "DolphinQt2/MenuBar.h"
@@ -94,6 +95,7 @@ private:
   void InitCoreCallbacks();
 
   void StartGame(const QString& path, const std::optional<std::string>& savestate_path = {});
+  void StartGame(const std::string& path, const std::optional<std::string>& savestate_path = {});
   void StartGame(std::unique_ptr<BootParameters>&& parameters);
   void ShowRenderWidget();
   void HideRenderWidget();