Wizards: Support using #pragma once instead of include guards

Allow users to choose #pragma once instead of #ifndef include guards in
generated header files.

Fixes: QTCREATORBUG-12166
Change-Id: I3ba41c7570beb9c5958e174b5581fcc25855050f
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Filip Bucek <fbucek@atlas.cz>

src/plugins/cpptools/abstracteditorsupport.h

@@ -49,6 +49,7 @@ public:
     unsigned revision() const { return m_revision; }
 
     static QString licenseTemplate(const QString &file = QString(), const QString &className = QString());
+    static bool usePragmaOnce();
 
 private:
     CppModelManager *m_modelmanager;