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Fix Sugarizer issue #437 and send a PR on the dev branch.

Description of the issue:

Add a tutorial to the GridPaint activity following tutorial 9 in Sugarizer development tutorial: https://github.com/llaske/sugarizer/blob/dev/docs/tutorial_step9.md

The tutorial:

• [ ] Should display a welcome message to explain the activity (could reuse the help message describing the activity in Sugarizer activities List view)
• [ ] Should explain usage of each element in the UI
• [ ] Should be localized
• [ ] Should respect the Sugarizer UI, as explained in tutorial