Make Columns disregard the preferred width of GtkEntry.

Make Columns disregard the preferred width of GtkEntry.

On OS X GTK, it requests a preferred width that's way too large. I
think that's because that's based on its max_width_chars rather than
its width_chars (and I only set the latter). But I don't want to
actually reduce its max_width_chars, in case (either now or in a
future version) that causes it to actually refuse to take up all the
space it's allocated.