CompletionInfo: use gtk_bin_get_child()

Instead of storing the widget in the private structure, we can simply
use gtk_bin_get_child().
Deprecate gtk_source_completion_info_{get,set}_widget()

If one day the CompletionInfo becomes more complex (like it was in the
past, with a ScrolledWindow and a Viewport), it will still be possible
to override gtk_container_add() and gtk_bin_get_child().
Don't use gtk_source_completion_info_{get,set}_widget()

It's less convenient to use the GTK+ functions, but it's better to have
a smaller API.