Autocomplete features

[waseemdev]

Hi,
Here are two commits:

1. I made Autocompleter abstract class and renamed default one to DefaultAutocompleter, so user can create its own implementation

   • also created SuggestionItem to be more customizable than just String, in some cases the displayed text in the popup list can be different than the actual code.

2. made the popup list customizable so user can use his own widget using autocompleteListBuilder.

   • also moved these variables (caretPadding, autocompletePopupMaxHeight, autocompletePopupMaxWidth) to be fields in CodeEditor widget