WidgetDefinitionReader is responsible to read the definitions from
an xml file. The first implemented definitions are style colors.
They are read from the file and stored into class fields.

This also adds the unit test which tests that the reader is
functioning as expected for a small certain subset of colors.

Change-Id: Icd44cb465b084c32db8323e2f2f7dfa57823d559
Reviewed-on: https://gerrit.libreoffice.org/68642
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>