Implement visibility_condition for Node3D and CanvasItem.

[EMBYRDEV]

Implements godotengine/godot-proposals#3433.

Also makes CanvasItems visibility testing a little cleaner.

I have seen #56446 and believe that this is a less intrusive solution to the problem of level design markup.

The "Runner Only" mode may also be useful for things like intrusive post-processing/fog-volumes/effects that you might not always wish to see in-editor.

When a node is hidden in the editor, it will show with an open eye that is greyed out, signifying to the user that the node is actually marked visible but that is overriden at the moment.

[dalexeev]

There should probably be a better name (visibility_mode?). visibility_condition sounds like a custom string expression that is evaluated to determine if a node should be shown.

[EMBYRDEV]

visibility_mode

That was also my first name choice but some nodes such as TouchScreenButton already use visibility_mode. I wanted to minimise breaking changes with this PR.

[EMBYRDEV]

I have added a debug option that allows developers to view editor only nodes in the running project.

[dalexeev]

but some nodes such as TouchScreenButton already use visibility_mode

Yes, in this case we are forced to choose some other name. Perhaps there is another option that is still better than visibility_condition. For example visibility_flags (but this also changes the enum values). Or something different.

[EMBYRDEV]

Appologies for the commit spam, still working this all out, I will close and reopen a squashed PR when I this passes checks.

[dalexeev]

New PR: #77301.