Improved ThemedWindow event handling

Improved ThemedWindow event handling by indicating whether an event was
handled or not. In the case of a DrawableCanvasControl the canvas is
given the opportunity to handle unhandled events.
Several enhancements added to HistogramEditor

Several enhancements were added to the HistogramEditor editor, including
allowing the user to edit many plot parameters such as bar color. Also
added the ability to notify application code when a histogram bar is
moused over or selected by clicking on it.