Added a node_added signal to the SceneTree

[NathanWarden]

This is a signal to accompany SceneTree.node_removed. It has the same behavior as node_removed only that it emits when a node is added.

Backstory, if needed:

This solves a major real world problem I'm having for a reusable library I'm writing where when an arbitrary code file, that another arbitrary developer adds to the scene, my only available options for them to use my library are:

1. Have developer add a block of code to every code file that needs to use my library. IE: Lots of repeated code = bad programming practice (For example, I'm wanting to port a project that has about 750 code files, I would have to repeat this line in most of them)
2. Have my library search the entire hierarchy every frame to see if their arbitrary code was on a new node added to the scene = very bad programming practice

Currently, there seams to be no good programming practice available in Godot to see when a node has been added to the scene tree, but there is one for when a node is removed, thus this pull request.

Thanks :)

[ghost]

Have you checked this issue: #11260?

[NathanWarden]

Yes, I saw it and also discussed in the help section on Discord why node_removed exists and why this should or should not exist. Nobody responded to me about this in the engine dev section. The discussion I had leads me to the conclusion that if this should not exist, then node_removed should not exist. Not having this leads to poor programming practice, just as not having node_removed would be possible, but would lead to bad programming practice. Therefore, I believe this signal should exist.

However, the issue you mention is actually fairly unrelated because this PR would be part of the SceneTree and is a more global and arbitrary event. A node is a local/concrete thing that you can setup while you're setting up your scene. SceneTree has no idea what will be under it, nor can you access it in the editor to add such a signal.

IE. For a node being setup in a scene you can obviously keep track of when nodes are added and deleted though code and custom signals, but for certain things, like in my case, you really can't do this in the SceneTree without violating well established programming practices, as I described above. This is why I assume node_removed is there, because you can technically track when a node is deleted through code at a global scale without the node_removed signal, but you're also forced to use bad programming practices like scanning the entire tree at a regular interval and tracking the state of every node and it's children. Likewise, if I need to know when nodes are arbitrarily added by an arbitrary programmer (I'm making a 3rd party library), my only option is to force them to violate well established programming practices (lots of repeated code), or use bad programming practices myself (search the entire tree every frame). Or, I could have the person consuming my library do a few hours of setup work that could have just be automatically handled... but only if this signal exists.

Thanks :)