Custom Autocompletion entries

[LeonMatthes]

As the developer of a Squeak Tool, it would be great to have the ability to define my own Autocompletion entries so that for example search bars could have custom fitted Autocompletion support.

Goal:
Define an API that allows TextMorph-model programmers to provide their own entries to the Autocompletion.

Secondary Goal:
Keep the API generic enough, so that anyone could implement the API hooks without instantly adding the Autocompletion as a dependency

Implementation notes
It is already possible to select a custom ECModel subclass, maybe just provide an ECModel subclass that asks the TextMorph-model for the necessary entries

[LeonMatthes]

Note:
Currently both ECMenuMorph, ECContext, ECModel and even ECController depend heavily on one another.
But for custom Autocompletion entries the ECContext would actually not be necessary, as it is just responsible for figuring out the Smalltalk context of where the completion occurs.
For a non-smalltalk text field this is of course not necessary.