title: How to add spacer in GTK+3?
body: <p>I am using C GTK+3, using Glade for the GUI design. I have a GtkGrid with 1 row and 3 columns. I have a button on column 2 and I want to add a spacer in column 1 and 3 so the button stays in the middle of the window. 
  Which GTK3 widget should I use to act as a simple 'spacer'?</p>