Review and improve GTK+ 3 porting workflow

[quozl]

https://github.com/sugarlabs/sugar-docs/blob/master/src/gtk3-porting-guide.md#how-to-port-a-sugar-activity-to-gtk-3 has a brief workflow for porting activiites.

@yashagrawal3, you may be interested in reviewing and improving this workflow.

Reference: http://meeting.sugarlabs.org/sugar-meeting/2018-05-11#i_2924905

[AtharvaParkale]

I would like to work on this issue.

[chimosky]

Hi @AtharvaParkale, feel free to open a PR with your changes.
You can also check out our CoC.