Better hook for completion engine in RubSmalltalkTextEditor

[Ducasse]

Right now we can specify at the class level the class of an engine.

RubSmalltalkEditor completionEngineClass: CoCompletionEngine.

This forces us to define a completion engine class for each configuration.

We would like to specify on the class RubSmalltalkEditor instance of an engine
this way we could configure and engine and that all the Rub instances uses it.

[Ducasse]

The instance creation logic is right now

completionEngine

	^ completionEngine ifNil: [
		CompletionEngineClass ifNotNil: [
			self completionEngine: CompletionEngineClass new.
			completionEngine ] ]

[Ducasse]

#17896