Add info on main editor when blank

At the first login or on an empty project, the user should get some help / clues to know how to start, where to click.
When the main editor is blank, display a placeholder sentence to explain what to do.

Suggestion: "Start your customization by choosing in the left panel the template you want to adapt. Then, drop elements in it and configure the fields as you need."

We will improve later if sentence needs to be too big: add a help button that display on overlay on the screen and explains the areas, or even just a static image to explain the areas and steps.