Add a tutorial to the PhysicsJS activity

[llaske]

Add a tutorial to the Physics 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 button in the toolbar
• Should be localized
• Should respect the Sugarizer UI, as explained in tutorial

[llaske]

Fixed in #521