Qt's comboboxes are controlled by models, there's no way around
that. To customize the chart-selection widget this must therefore
be abstracted into a model. On the upside, this hopefully can
be used for desktop and mobile.
The model provides icons and paints a warning-symbol on it
if the statistics core code deems the chart to be not recommended.
Notably, when plotting a categorical bar chart against a
numerical value (in such a case histograms are preferred).
Includes a fix for a silly oversight in CMakelist.txt: add the
statstranslations.h header.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
