Deprecate the GtkSourceCompletionInfo::before-show signal

It was used as a hack in gtksourcecompletion.c, but it's no longer the
case.

https://bugzilla.gnome.org/show_bug.cgi?id=696506
Hide CompletionInfo when focus-out-event on the attached-to widget

The application just needs to attach the calltip window to the
GtkTextView widget, so when the text view looses focus, the calltip is
hidden.
Completion utils: small clean-up