Add coroutines await() to signals

[CedNaru]

• Add a new godot.couroutine module.
• The Gradle plugin can now optionnally include it (and implicitly kotlinx.coroutines)
• Add a Godot specific scope for coroutines
• Add an await() functions to all signals that will suspend the coroutines until the signal is emitted.
• await() can return the signal's arguments, directly (when 1), as a tuple (when 2 and 3), and as list (4+)

I also changed the package of Signal and Callable (I needed them for the await implementation so I took the opportunity).
So far everything has been in the same godot.core package except for those 2. Maybe it's an error and we should split everything into smaller packages but right now I wanted to focus on more consistency.
Note that the await() functions are in the godot.coroutines package as it's not considered the same library.

[chippmann]

Did you check that none of the IDE checks are requiring the signals to be in the signal package?

[chippmann]

What i meant with tests which switch coroutine context is that you await a signal in a test using withContext(Dispatcher.SomeOtherdispatcherThanYouStartedWith) { signal.await() } But that can be added later as well.

Other than that I'm happy :-) Good first implementation with which we can start with and have a look how it feels.