Introduce multi-touch gestures in onboarding #175

Problem

elementary's multi-touch gestures are fantastic and essential for everyday use.

However, many users may not be using this functionality because they don't know it exists in elementary, unless they go to system settings.

Proposal

Show in onboarding, a brief instruction on using multi-touch gestures.

Just as an example, I took some Mac images that show how to use the gestures:

Prior Art (Optional)

No response