For some chart (e.g. pie charts or stacked bar charts), we want
to display a legend. QtCharts' legend interface happens to be
private and therefore is of no use.
This introduces a legend box which is implemented using
QGraphicItems, which can be placed on top of QCharts. It's very
unfancy, but works for now. If there are too many items, not
all are shown. Currently, the legend is configured to fill
at most half of the width and half of the height of the chart.
This might need some optimization.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
