Add support for using HarfBuzz instead of ICU LE

HarfBuzz is an up to date, actively maintained OpenType layout engine,
while the ICU LayoutEngine we use now has been unmaintained for a
while now, and existing users are encouraged to switch to HarfBuzz.

This is work in progress, text layout mostly works, but the handling
of combining marks is broken and needs further work, so it is kept
optional for now, with SAL_USE_HARFBUZZ env variable to enable it at
runtime.

Change-Id: I07e698f7486379bae68329771695cd94d6e561b5
Reviewed-on: https://gerrit.libreoffice.org/3518Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>