Help: Provide a button for re-indexing the documentation

It sometimes just breaks, so having a simple way for re-indexing seems
appropriate.

Task-number: QTCREATORBUG-11484
Change-Id: I4010d31cbe04f9ef57ebe772610118d8aa7a2ea2
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>