In the promises, we use a wrapper Eina_Promise_Cancel_Cb to
invalidate the wrapper if it ever gets cancelled from outside. When
invalidating from C#, we can do it directly.
For the futures, likewise, in order to be able to invalidate the
wrapper when the chain it belongs to gets resolved we then() an
internal future with a callback to invalidate the wrapper we return to
C#. The return of this intermediate then() is the future we actually
return to the user.
Also added ECANCELED to the list of default eina.Errors
Depends on D6173