CppTools: Rename "Update Code Model" to "Reparse Externally Changed Files"

It was not clear what "Update Code Model" did.

Change-Id: I245307039f8770047b80c7234481099edfe60bc9
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>