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/DiscIO/Enums.h

@@ -10,7 +10,7 @@
 
 namespace DiscIO
 {
-// Increment CACHE_REVISION (GameListCtrl.cpp) if these enums are modified
+// Increment CACHE_REVISION (GameFileCache.cpp) if these enums are modified
 
 enum class Platform
 {
@@ -68,6 +68,9 @@ enum class Language
   LANGUAGE_UNKNOWN
 };
 
+std::string GetName(Country country, bool translate);
+std::string GetName(Language language, bool translate);
+
 bool IsDisc(Platform volume_type);
 bool IsWii(Platform volume_type);
 bool IsNTSC(Region region);
@@ -82,5 +85,5 @@ Country CountrySwitch(u8 country_code);
 Region GetSysMenuRegion(u16 title_version);
 std::string GetSysMenuVersionString(u16 title_version);
 
-std::string GetCompanyFromID(const std::string& company_id);
+const std::string& GetCompanyFromID(const std::string& company_id);
 }