Histogram bars

This pull request allows to plot histogram as bars instead of lines, for Int data only (Byte, Int16 and Int32).

This is much more natural for visualizing histograms, as it removes any gaps in the histogram.

Works with qwt5 (imported code from qwt5 source) and qwt6 (directly).

It also delegates the number of bins and other parameters to the data provider. Histogram bin count is also dependent on min/max in Int16/Int32.