Port to Python 3

[quozl]

Initial port by @pro-panda was held in python3 branch.

This collection of patches fixes several problems;

• scale the data pane width, fixes https://github.com/sugarlabs/chart/issues/4,
• chart type buttons were occasionally unreliable (clicked vs toggled signal),
• read and write; data retention, ordering, and title,
• logging ambiguities,
• add to save as image name.

[quozl]

Thanks. @Aniket21Mathur, would you like to test?

[Aniket21mathur]

Thanks. Reviewed. Tested. :-)