WiiSaveBanner: fall back to $userdir/Load/WiiBanners

Unlike custom banners which work as an override, this mechanism works as
a fallback. The use case is if you have games you don't really play but
want to keep around for testing purposes without filling up your NAND
with lots of saves. For ease of use, the directory structure is the same
but only title/$title_hi/$title_lo/data/banner.bin files are
relevant.

Source/Core/Common/NandPaths.h

@@ -14,6 +14,7 @@ enum class FromWhichRoot
 {
   Configured,  // not related to currently running game - use D_WIIROOT_IDX
   Session,     // request from currently running game - use D_SESSION_WIIROOT_IDX
+  Banners,     // fallback for Wii savegame banners - use D_BANNERS_WIIROOT_IDX
 };
 
 std::string RootUserPath(FromWhichRoot from);