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/NetPlay/NetPlayDialog.cpp

@@ -532,7 +532,7 @@ std::string NetPlayDialog::FindGame(const std::string& game)
   return RunOnObject(this, [this, game] {
     for (int i = 0; i < m_game_list_model->rowCount(QModelIndex()); i++)
     {
-      if (m_game_list_model->GetUniqueID(i).toStdString() == game)
+      if (m_game_list_model->GetUniqueIdentifier(i).toStdString() == game)
         return m_game_list_model->GetPath(i).toStdString();
     }
     return std::string("");