completion: use new GtkTreeColumn for placeholder text and align to it.

The little bit of magic is the horizontal_separator and focus_padding
values which were determined by reading the layout code of gtktreeview.
If the column was the "expander-column", it would be a bit more complex
but we don't have those thankfully.

https://bugzilla.gnome.org/show_bug.cgi?id=706465