Add HarfBuzz-NG support

Some features are of limited usefulness for now (same as with HB-old):
* mixed scripts cases aren't handled correctly due to an outdated
  script and bidi itemization implementation;
* language-by-script detection: the only fallback to locale's LANG is used

Some features are missing entirely (in compare to HB-old):
* justification points support: not implemented in HarfBuzz-NG

Task-number: QTBUG-18980
Task-number: QTBUG-14590
Task-number: QTBUG-16128

Change-Id: Ic98a10054be5fac55224ef31c7261168c0bf8739
Reviewed-by: Lars Knoll <lars.knoll@digia.com>