add default onboarding text to timeline

As the 3d edit mode now has a good default text and way to add the correct import / prompt the user towards what they should do, we should also add a good default text to the timeline tab when there is no timeline in the scene. I've attached the screenshots from the new snapshot and the concept design for the timeline panel, the icon as png and the text is as follows.

No timeline in this file yet!

To create an animation you need to add a timeline, click the plus button on the top left of the panel to create a default timeline.

Once you have a timeline you can edit the settings by opening the timeline dialog button (ICON)