CppEditor: Trigger "Convert to Pointer" only in function scope

Change-Id: I449330e9d5811f88b359c82726154065b3a6b494
Task-number: QTCREATORBUG-12866
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>