Rework scene restoring

[KoBeWi]

Changed the way scene restoring works. Instead of loading all scenes, only the current scene is loaded, other scenes are opened as "dummy scenes". They are not being loaded until you switch to them. This makes the editor open fast regardless of how many scenes you had opened when closing.

godot.windows.editor.dev.x86_64_9fRMa1xSUw.mp4

As a result, restore_scenes_on_load setting became obsolete and I removed it.
Discussion for more context: https://chat.godotengine.org/channel/editor?msg=6MqBpHYC5xdJShs4z
Needs some testing.

[KoBeWi]

This part needs to be improved to include some basic error handling.

[Lielay9]

https://xkcd.com/1172/
I haven't necessarily disabled restore_scenes_on_load for performance reasons, I just prefer to start without previous scenes loaded - the disabled behavior defaulting to opening the main scene has been a happy accident. On toy-projects with single or couple scenes, the relevant scene opens automatically, and on bigger projects I can mentally reset after every session and start a day from scratch. On some older projects it's also helpful at locating the main scene to start the process of retracing the project scene hierarchy. Finally the setting has eased editing tool scripts in-editor after some idiot from a minute ago bricked the scene by writing an infinite loop.

Could we keep the toggle or better yet make a new property with 3 options: previous, main, none?

[KoBeWi]

I restored the setting, but changing how it works is out of scope for this PR.

[AdriaandeJongh]

If async loading of the scenes speeds up the editor, then we should totally do this – or possibly an iteration on this where the remaining scenes are loaded asynchronously in the background.

I coincidentally didn't have any heavy-to-load scenes in the projects on my computer lol, but can you elaborate whether clicking on unloaded scenes also pops some sort of progress bar if it takes a little while? The hang in your video doesn't look great. We'd like to avoid 'software is unresponsive' kind of situations I think?

[KoBeWi]

I forgot this exists 🤔
Some progress bar makes sense, but I'd wait until #116905 is merged until I make further improvements.