GtkComboBoxText: allow id='' tag in builder markup

GtkComboBoxText: allow id='' tag in builder markup

Presently, GtkComboBoxText lets you list the items for it in the
GtkBuilder markup like so:

<items>
  <item translatable='yes'>Red</item>
  <item translatable='yes'>Blue</item>
</items>

but has no way to set the "id" column for added items.  Add an id='' tag
for that purpose so that you can do:

  <item id='red' translatable='yes'>Red</item>

https://bugzilla.gnome.org/show_bug.cgi?id=667319