printers: Set min-content-height on the scrolled window

Set a minimum content height of 490px for the panel when the
allocated height is smaller than 490px.

490 is an estimated value for the panels to properly fit on netbook
screens. See https://wiki.gnome.org/Design/SystemSettings#Notes

https://bugzilla.gnome.org/show_bug.cgi?id=767600

panels/printers/printers.ui

@@ -46,6 +46,8 @@
     <child>
       <object class="GtkScrolledWindow">
         <property name="visible">True</property>
+        <property name="hscrollbar-policy">GTK_POLICY_NEVER</property>
+        <property name="min-content-height">490</property>
         <child>
           <object class="GtkBox" id="content">
             <property name="visible">True</property>