WiiRoot: Move content manipulation to separate functions

{Initialize,Shutdown}WiiRoot should only be responsible for setting the
SESSION_WII_ROOT or managing the temporary NAND directory.

Move all the content manipulation out of these functions to ensure
separation of concerns and call them after/before WiiRoot init/shutdown
to make sure they operate on the correct root.

Source/Core/Core/HW/HW.cpp

@@ -54,11 +54,13 @@ void Init()
     Core::InitializeWiiRoot(Core::WantsDeterminism());
     IOS::Init();
     IOS::HLE::Init();  // Depends on Memory
+    Core::InitializeWiiFileSystemContents();
   }
 }
 
 void Shutdown()
 {
+  Core::CleanUpWiiFileSystemContents();
   // IOS should always be shut down regardless of bWii because it can be running in GC mode (MIOS).
   IOS::HLE::Shutdown();  // Depends on Memory
   IOS::Shutdown();