Core: Use a shared_ptr for i18n categories.

This does not make them thread safe, but it reduces the chances of a crash
a bit (see #12594.)

UI/DisplayLayoutScreen.cpp

@@ -231,9 +231,9 @@ void DisplayLayoutScreen::CreateViews() {
 
 	using namespace UI;
 
-	I18NCategory *di = GetI18NCategory("Dialog");
-	I18NCategory *gr = GetI18NCategory("Graphics");
-	I18NCategory *co = GetI18NCategory("Controls");
+	auto di = GetI18NCategory("Dialog");
+	auto gr = GetI18NCategory("Graphics");
+	auto co = GetI18NCategory("Controls");
 
 	root_ = new AnchorLayout(new LayoutParams(FILL_PARENT, FILL_PARENT));