Financial data, add option to remove gaps for discontiguous data

This thread corresponds to feature request #97 and I was directed to post it here.

Here's an idea for how I think we can improve QwtDateScale engine:

Provide method to:

• optionally drop gaps in QwtDateScale during its calculations so that no gaps display in the chart
• Bonus Points: provide method to set a fixed pixel width for the gap between bars for QwtPlotTradingCurve

I'm primarily concerned about use cases for financial data which has gaps in its timeseries data throughout.

I'll test it thoroughly on macOS and Win10 if any changes are made.