Order proposals alphabetically

[fm-117]

Much like #2723, the goal here is to ensure that the completion proposals are always ordered alphabetically.
Current implementation uses declaration order. Note that the order applies to list of same type elements: for example after PERFORM we dould like to return the list of paragraphs ordered alphabetically, followed by the list of sections ordered alphabetically and finally the list of numeric variables also ordered alphabetically.

Apply refactoring where needed to avoid code duplication.