view: implement GtkBuildable::get_internal_child() for "completion"

To be able to configure the completion directly in the .ui file, like it
is done in test-completion.

https://bugzilla.gnome.org/show_bug.cgi?id=736367
completion: implement GtkBuildable::get_internal_child()

For the info_window.
CompletionInfo: set a border width of 3

So that the label or another widget has some space around it in the
calltip.

Since the main completion window is also a CompletionInfo, set the
border width to 0 for it.