Make the presentation editable in the editor, with Drag and Drop

[NathanLovato]

When I first designed this project, Godot had some limitations that made scenes a pain to use to design slides. I just ported the slides' data to JSON, and this text-based approach isn't too great.

Now the UI system is solid, and with upcoming improvements in 3.1, it's worth making it more reusable. The idea is to let you compose slides from the editor, using premade UI widgets (or by adding your own):

And to order the slides you'll just add them and order them in a new Presentation:

The reasons:

• When you move pictures or files in the project, Godot will resolve dependencies for you
• It's a lot more flexible
• It's a lot more fun!
• It'll be easier to update the presentation or add new ones
• We can use the built-in translation system!
• It's also a great way to showcase Godot's UI system when you give the presentation. E.g. you should be able to drag and drop a new .theme file on the presentation node and the presentation's look should update instantly

[NathanLovato]

Done in the drag-and-drop-branch