Builder interface for registration of Signals

[chitoyuu]

Since v0.9, we have had idiomatic builder interfaces for registering properties, then for methods, but not yet for signals. This makes signals a lot more cumbersome to use, and out of place in terms of API design, compared to other aspects of exposing a GDScript API.

Drawbacks

No known drawbacks. PropertyBuilder and MethodBuilder has proven to be successful.

Alternatives

No known alternatives. The builder pattern is ubiquitous in the Rust ecosystem for use cases like this.

Further features like typed signal emission (#758 (comment)) and derive macro support (#715) are orthogonal to the procedural registration API.

Prior art

The API of clap can serve as an inspiration on how multiple named arguments can be handled.

[Bromeon]

Implemented in #828.

The ClassBuilder::signal() API is available online.