bnc#812796: Correctly handle static value array for OOXML charts.

We need to pass the role of the data sequence in order to avoid unreliable
guess work when importing static value array.

Also, not all Excel's scatter plots have real numeric X values; some have
textural X values in which case Excel switch to generating 1, 2, 3, ... as
X values.  When importing to our chart implementation, using "categories" role
in such cases instead of "values-x" results in a more faithful chart rendering.

(cherry picked from commit 6c4e21a234f12e1310ba06f9859e08b424acf8bf)

Conflicts:
	chart2/source/inc/InternalDataProvider.hxx
	chart2/source/tools/InternalDataProvider.cxx

Change-Id: If4bc1f650bb024dcd1b1b36537f457fb38404a78
Reviewed-on: https://gerrit.libreoffice.org/10040
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>