It's nice to indicate when a title is truncated with an ellipsis.
Because themes may draw a title multiple times to draw a shadow, or
may include the window icon within the title area, we can't determine
the proper ellipsization width automatically, so add an optional
attribute to the <title/> element "ellipsize_width" which, if set,
is the width to ellipsize at.

This is only enabled if a theme version of 3.1 is required.

When it's not set, we keep the old behavior of just letting the
title be clipped with a hard edge.

https://bugzilla.gnome.org/show_bug.cgi?id=591842