Add @Signal Macro

[samdeane]

Implements #586.
See also previous discussion on #584.

TL;DR

This PR adds a new @Signal macro, giving a cleaner and more consistent way to add user-defined signals from Swift.

@Godot
class MyNode: Node {
  @Signal var mySignal: SimpleSignal
  @Signal var signalWithArgs: SignalWithArguments<Int, String>
}

These signals can be used in the same way as built-in signals:

node.mySignal.connect { /* do something */ }

node.signalWithArgs.connect { intArg, stringArg in
  // do something
}

node.mySignal.emit()
node.signalWithArgs.emit(1, "foo")

Motivation

The existing #signal macro defines the signal as a static property on the class that contains it.

This is different from the generated implementation of built-in signals, and means that we have to use a different syntax to work with them -- which feels... non-optimal.

Implementation

We add a new @Signal macro expansion which generates a computed property on the class, using GenericSignal.
This is the same approach that is used to implement the signals imported from the Godot API.

We also add an emit() method to GenericSignal, which is of course essential to support emitting signals defined in Swift.

We add SimpleSignal and SignalWithArguments as slightly clearer aliases for GenericSignal

This change should be 100% additive, and should not break the old #signal macro.

To Do

If this change is adopted, it's probably worth updating the documentation and deprecating the old macro.

[migueldeicaza]

This is lovely! Would you mind testing the Object.callv approach? If it works, that might be better to maintain in the long run.

[samdeane]

Using Object.callv seems to work well.
Have also squashed all the commits for a cleaner history.

[migueldeicaza]

Thank you!

Merging.