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/Core/WiiUtils.cpp

@@ -160,6 +160,28 @@ bool InstallWAD(const std::string& wad_path)
   return InstallWAD(ios, DiscIO::WiiWAD{wad_path}, InstallType::Permanent);
 }
 
+bool UninstallTitle(u64 title_id)
+{
+  IOS::HLE::Kernel ios;
+  return ios.GetES()->DeleteTitleContent(title_id) == IOS::HLE::IPC_SUCCESS;
+}
+
+bool IsTitleInstalled(u64 title_id)
+{
+  const std::string content_dir =
+      Common::GetTitleContentPath(title_id, Common::FromWhichRoot::FROM_CONFIGURED_ROOT);
+
+  if (!File::IsDirectory(content_dir))
+    return false;
+
+  // Since this isn't IOS and we only need a simple way to figure out if a title is installed,
+  // we make the (reasonable) assumption that having more than just the TMD in the content
+  // directory means that the title is installed.
+  const auto entries = File::ScanDirectoryTree(content_dir, false);
+  return std::any_of(entries.children.begin(), entries.children.end(),
+                     [](const auto& file) { return file.virtualName != "title.tmd"; });
+}
+
 // Common functionality for system updaters.
 class SystemUpdater
 {