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/plugins/cpptools/refactoringengineinterface.h

@@ -57,8 +57,7 @@ public:
                                     CppTools::ProjectPart *projectPart,
                                     RenameCallback &&renameSymbolsCallback) = 0;
     virtual void startGlobalRenaming(const CursorInEditor &data) = 0;
-
-    virtual bool isUsable() const = 0;
+    virtual bool isRefactoringEngineAvailable() const { return true; }
 };
 
 } // namespace CppTools