Replace `@Cancelable` by `ICancellableEvent` interface & related changes

[Technici4n]

• Replace the annotation by an interface that brings the isCanceled and setCanceled methods in scope for cancellable events only.
• Change IEventBus#post to return the event itself.
• Replace GenericEvent by Event & IGenericEvent bound in a few places - this is necessary to ensure that the cancellation check is properly removed for non-cancellable events.
• Standardize spelling to most common US english variant. I chose the most common US spelling for: canceled (single L), cancellable (double L), and cancellation (double L).
  • Importantly, this does not change isCanceled() and setCanceled().

[Technici4n]

Generic events are now deprecated for removal, but I didn't want to make this dependent on the capability rework which still uses them and might take a bit of time to land. It's a bit messy to have the casts like in this PR, but they work and we'll be able to clean that up later.

I'm not totally set on removing the ASM handler yet (I need to benchmark more), so I figured I'd already get this out of the way.