* import static text & fields:
   VALUE, SERIESNAME, COLUMNNAME
* text is formatted
* DataPointCustomLabelField with field type
  (DataPointCustomLabelFieldType) was introduced.
* text can have many portions & multiple lines
* unit tests for import data labels with formatting

Not implemented: CELLREF field support which needs
importing some additional data from extLst

Shows custom text as a label for data points.

Change-Id: Iba8fd508eb16356b05586b93d7b8da32240d2b91
Reviewed-on: https://gerrit.libreoffice.org/48243Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Szymon Kłos <szymon.klos@collabora.com>