Center Global Config after window resize

Assure Global Config window will remain on the center of the screen
after window is resized.

Author: alimirjamali

qubes_config/global_config.glade

@@ -6,7 +6,7 @@
   <object class="GtkWindow" id="main_window">
     <property name="can-focus">False</property>
     <property name="title" translatable="yes">Qubes OS Global Config</property>
-    <property name="window-position">center</property>
+    <property name="window-position">center-always</property>
     <child>
       <object class="GtkBox" id="background_box">
         <property name="visible">True</property>