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/Config/ARCodeWidget.cpp

@@ -16,10 +16,10 @@
 #include "Core/ConfigManager.h"
 #include "DolphinQt2/Config/CheatCodeEditor.h"
 #include "DolphinQt2/Config/CheatWarningWidget.h"
-#include "DolphinQt2/GameList/GameFile.h"
+#include "UICommon/GameFile.h"
 
-ARCodeWidget::ARCodeWidget(const GameFile& game)
-    : m_game(game), m_game_id(game.GetGameID().toStdString()), m_game_revision(game.GetRevision())
+ARCodeWidget::ARCodeWidget(const UICommon::GameFile& game)
+    : m_game(game), m_game_id(game.GetGameID()), m_game_revision(game.GetRevision())
 {
   CreateWidgets();
   ConnectWidgets();