Allow opening scenes with missing scene dependency

[KoBeWi]

When a scene has instances of scenes that no longer exist in the project, you won't be able to open them, with the well-known "Scene invalid/corrupt" error. This PR fixes that.

QdRyle2wEe.mp4

However, while it technically works (i.e. you can open the scene), it could use some improvements. I wanted it to at least display path of the missing scene, but I wasn't sure how to obtain it (in the video it shows as TODO). Also, contrary to what the tooltip says, saving the scene will result in data loss. The instance will be gone and any editable children data is lost. It would be nice to preserve at least the instance; if it's not possible/too difficult, changing the tooltip would be fine for now too.

[Mickeon]

Would be nice to preserve at least the instance

It would be nice if InstancePlaceholder were capable of doing that.

[Zireael07]

@Mickeon Something for a followup PR I think?

[Mickeon]

Yeah. For the focus of this PR, changing the tooltip to not blatantly lie may be good enough.

[Mickeon]

Perhaps MissingNode could have a Dictionary containing NodePaths pointing to the lost properties and values. When loading a PackedScene in the editor, it is detected and rebuilt if possible, or left as is. It sounds extremely hacky but... Yeah.

[KoBeWi]

I managed to make the path available using a questionable hack 😬 Though for some reason it's not always reliable (it seems to depend on some cache).
I also adjusted the message:

If the path can't be determined for some reason, there is a slightly different massage that node is unknown. For now the instance is completely discarded.

In total there are 3 paths where loading scene can fail: instances, placeholders and inheritance. The former 2 are handled by this PR, the last one I think is not recoverable.

[reduz]

I think this looks good but keep in mind sometimes dependencies fail to open because they miss resources and they should still be openable anyway, so maybe this should only really be used in case the scene is not found.

[akien-mga]

Will need a rebase.

[KoBeWi]

Rebased. Seems like someone added a description for recording_properties in the meantime. I replaced it with mine, as it provides more information.

[reduz]

give me a second before merging this, I want to check something

[reduz]

Alright, I think its good to merge, but I think what we still need to have an option to recursively fix dependencies on subresources that don't load because of this.

[akien-mga]

Thanks!

[Flavelius]

I tried to address something like this a while ago aswell, and while it was more of a hackfix (#75103), i think not losing nodes is still the actual desire, as some sources might just have been renamed or moved; where it shouldn't be assumed they are actually missing.