DolphinWX: Set C++ locale

After 3a83ebc, the Show System Clock feature started using the
unfortunate combination of MM/DD/YY dates (rare outside of the US)
and 24-hour time (rare in the US) regardless of the user's locale
settings. This commit makes it use the configured locale again.

I've noticed one minor difference in behavior between now and
before 3a83ebc: The new way of setting the C/C++ locale seems to
treat "en" as "en-US", but the wx way of setting the C locale
treated it as "en-GB" (at least on Windows).

Source/Core/UICommon/UICommon.h

@@ -19,6 +19,10 @@ void EnableScreenSaver(Display* display, Window win, bool enable);
 void EnableScreenSaver(bool enable);
 #endif
 
+// Calls std::locale::global, selecting a fallback locale if the
+// requested locale isn't available
+void SetLocale(std::string locale_name);
+
 void CreateDirectories();
 void SetUserDirectory(const std::string& custom_path);