This adds the following:
- Retrieving of features using HarfBuzz
- Retrieving of feature definitions from graphite fonts
- Adds description texts of OpenType features
- Tests for retrieving features (Linux Libertine G graphite font)
  and making sure the features and definitions are what we
  expect.

Change-Id: I51573fe086e59c6228cb546bbfac95ac53824c47
Reviewed-on: https://gerrit.libreoffice.org/55892
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>