Variable scope error

Introduced when I uncritically followed cppcheck's advice.
Displaying annotations
Putting Interval in its own file
PlotViewWidget now subclass of QwtPlot

The previous configuration added in another nested QWidget, to no
purpose.
Better comparisons

Comparisons are now done in a better way, with a proper dialog
interface. The comparison code is improved somewhat, but is still rather
ad-hoc.