CustomWizard: Support current date and time in default texts

This provides the functionality for the documented, but not working,
%CurrentDate% and %CurrentTime%.
Changes these two to locale independent format YYYY-MM-DD and HH:MM:SS.
Also adds explicit variants %CurrentDate:ISO%, %CurrentTime:ISO%,
%CurrentDate:RFC%, %CurrentTime:RFC%, %CurrentDate:Locale% and
%CurrentTime:Locale% which format the date and time as described
by ISO 8601, RFC 2822 and the Locale respectively.

Task-number: QTCREATORBUG-9963

Change-Id: I39a5c0f3b606559bf4f5c5e40e421f338de4c5a4
Reviewed-by: Lorenz Haas <lykurg@gmail.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>