QDnsLookup: Support for custom DNS server

This commit only adds a new QProperty, "nameserver", to QDnsLookup.
This property currently does not do anything and is not used internally
by QDnsLookup. The next commit will implement it's usage.

Task-number: QTBUG-30166
Change-Id: I85b72bd6661603128cab4068c1b83883fb2bfd1a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>