Type-safe signals for engine classes

[Bromeon]

Following up #1000, this pull request exposes all signals declared in Godot engine classes.
Closes #1108.

For example, Node has the child_entered_tree signal:

To connect such a signal in Rust, we can simply do:

let mut node: Gd<Node> = /* some node in tree */;

node.signals()
    .child_entered_tree()
    .connect(|child_node| {
        println!("A child node entered the tree: {child_node}");
    });

In other words, it's the exact same API as before.

This PR also comes with other small changes:

1. WithSignals trait is split into WithSignals (all signal-enabled classes) and WithUserSignals (only #[signal] user-defined ones).
2. Gd::signals() now takes &mut instead of &. This is mostly for consistency with self.signals(), as well as Godot's own APIs Object::connect, Object::emit etc. all operating on &mut self. Technically it makes not much difference, but signal manipulation being linked to mutation has a higher chance of detecting unintended changes.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1111

[Bromeon]

Next plan: making signals in base classes accessible. At the moment, Gd<T>::signals() only returns signals from the class T itself, but it should be possible to connect/emit those in superclasses as well.

Potential ideas:

• Duplicating signals (preferably avoided)
• Deref/DerefMut of the collection type to its base class' type
• Explicit selection à la obj.signals_of::<BaseClass>().some_signal()