This adds a font feature dialog activated on the Character page,
which adds a way to manipulate the available font features for the
current selected font (by adding them to the font name).

Change-Id: Icd0eda31b235d3cc8ef6eaee582fb78b4b346d78
Reviewed-on: https://gerrit.libreoffice.org/55894
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>