Updated Hungarian translation
Release 3.7.1
Doc: fix header to include for the word completion provider
CompletionModel: better format the prototypes
CompletionModel: rename three functions

I remember that when reading the code the first time it was difficult to
understand the purpose of those functions. Since the API is private,
there is no renaming problems.
Completion: more robust get_selected_proposal()

The header can't be selected, normally. But when proposals are
removed/inserted by the model, it can occur. In this case, the function
should return false to mean that no proposal is selected.