Common: Rename UTF16ToUTF8

This function does *not* always convert from UTF-16. It converts
from UTF-16 on Windows and UTF-32 on other operating systems.

Also renaming UTF8ToUTF16 for consistency, even though it
technically doesn't have the same problem since it only was
implemented on Windows.

Source/Core/WinUpdater/Main.cpp

@@ -28,7 +28,7 @@ std::vector<std::string> CommandLineToUtf8Argv(PCWSTR command_line)
   std::vector<std::string> argv(nargs);
   for (int i = 0; i < nargs; ++i)
   {
-    argv[i] = UTF16ToUTF8(tokenized[i]);
+    argv[i] = WStringToUTF8(tokenized[i]);
   }
 
   LocalFree(tokenized);

Source/Core/WinUpdater/WinUI.cpp

@@ -180,7 +180,7 @@ void ResetCurrentProgress()
 
 void Error(const std::string& text)
 {
-  auto wide_text = UTF8ToUTF16(text);
+  auto wide_text = UTF8ToWString(text);
 
   MessageBox(nullptr,
              (L"A fatal error occured and the updater cannot continue:\n " + wide_text).c_str(),
@@ -200,7 +200,7 @@ void SetCurrentProgress(int current, int total)
 
 void SetDescription(const std::string& text)
 {
-  SetWindowText(label_handle, UTF8ToUTF16(text).c_str());
+  SetWindowText(label_handle, UTF8ToWString(text).c_str());
 }
 
 void MessageLoop()
@@ -256,7 +256,7 @@ void LaunchApplication(std::string path)
 {
   // Hack: Launching the updater over the explorer ensures that admin priviliges are dropped. Why?
   // Ask Microsoft.
-  ShellExecuteW(nullptr, nullptr, L"explorer.exe", UTF8ToUTF16(path).c_str(), nullptr, SW_SHOW);
+  ShellExecuteW(nullptr, nullptr, L"explorer.exe", UTF8ToWString(path).c_str(), nullptr, SW_SHOW);
 }
 
 void Sleep(int sleep)