Opening big project should show some "loading" dialog

[KoBeWi]

Godot version:
3.1

Issue description:
When you open a not-small project and have "Restore scenes on load" enabled, you see this:

A totally empty project that lasts for few seconds. It's confusing and some people might get heart attack seeing their project empty. The only indication that something happens is the currently loading scene's name on title bar (and the fact that everything is frozen). IMO there should be at least some "LOADING" text in the middle and ideally a progress bar.

Steps to reproduce:

1. Open few big scenes
2. Enable "Restore scenes on load"
3. Open the project again

[KoBeWi]

btw, it got worse now XD The editor isn't completely frozen, you can click things, but you just get more empty dialogs and unresponsive options. Like, you can open "Quick Open Scene", which is empty until the project loads and you open the dialog again.

[KoBeWi]

Closing in favor of #47053, which is much more detailed.
btw this is not related to "Restore Scenes on Load", it just happens in big projects, as described in #47053