CppTools: add multiple refactoring engines support

Make model manager able to select the most functional
refactoring engine from the available ones.

Change-Id: I74031c910706fd694a0a7def022531501f1ea005
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/libs/utils/smallstringfwd.h

@@ -25,10 +25,10 @@
 
 #pragma once
 
-namespace Utils {
-
 using uint = unsigned int;
 
+namespace Utils {
+
 class SmallStringView;
 template <uint Size>
 class BasicSmallString;