[gnome-boxes/center-customize-button] wizard: Center the "Customize" button below

[Felipe Borges <felipeborges src gnome org>]

commit d8870a8f15fa1ff1593675c32550a31e2ab7cf0f
Author: Felipe Borges <felipeborges gnome org>
Date:   Mon Jan 29 17:48:53 2018 +0100

    wizard: Center the "Customize" button below
    
    The current placement of the "Customize" button makes it seems
    like the action is tied to the Disk property only.
    
    The wireframes for the new machine assistant already position the
    button in a row below, expanding horizontally to the size of the
    description text in the top.
    
    https://raw.githubusercontent.com/gnome-design-team/
    gnome-mockups/master/boxes/wires/newbox-assistant.png
    
    Fixes #71

 src/wizard.vala | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/src/wizard.vala b/src/wizard.vala
index 5182a7eb..f9d57ac4 100644
--- a/src/wizard.vala
+++ b/src/wizard.vala
@@ -755,7 +755,9 @@ public void append_customize_button (CustomizeFunc customize_func) {
             return;
 
         var button = new Gtk.Button.with_mnemonic (_("C_ustomize…"));
-        attach (button, 2, current_row - 1, 1, 1);
+        button.hexpand = true;
+        button.margin_top = 20;
+        attach (button, 0, current_row, 2, 1);
         button.show ();
 
         button.clicked.connect (() => { customize_func (); });