Completion: use a CompletionInfo for the main window

Now, to shrink the main window, we only have to shrink the child widgets.
The window will be resized automatically.

But for the moment the main window has still a fixed size.
Completion: move utils functions in CompletionInfo

Those functions are used only by CompletionInfo now.
Completion: move remaining completion_utils functions

The remaining utils functions were used only in Completion.