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.

Reviewed-on: https://gerrit.libreoffice.org/68642
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
(cherry picked from commit 3fc464ae)

Change-Id: Icd44cb465b084c32db8323e2f2f7dfa57823d559