Reorganize Options dialog

- Add Data Sources, Canvas/Legend sections
- Fix excess whitespace issue for list widget pane in QtDesigner and QGIS
- Switch GDAL drive table load to on current page name

Appreciate feedback on the choices, and any suggestions.

Slideshow of screen snaps (little taller dialog height, to show layouts in scroll areas):
https://www.dropbox.com/sh/l08y5xyxm20q5ev/aOLAIuoSVV