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/cpprefactoringengine.h

@@ -36,8 +36,6 @@ public:
                             CppTools::ProjectPart *projectPart,
                             RenameCallback &&renameSymbolsCallback) override;
     void startGlobalRenaming(const CppTools::CursorInEditor &data) override;
-
-    bool isUsable() const override { return true; }
 };
 
 } // namespace CppEditor