Make GtkItemEntry compatible with GTK+ 2.17.4 and later.

GTK+ 2.17.4 breaks GtkEntry's buffer into a separate class named
GtkEntryBuffer and eliminates the associated members from GtkEntry.
The GtkItemEntry code in lib/gtksheet attempted to use some of those
members directly, which no longer works, so this commit changes that
code to use public function interfaces instead.  Some of these public
function interfaces were introduced in GTK+ 2.17.4, so those pieces
are conditionalized on the version of GTK+ detected at build time.

This commit makes PSPPIRE compiled with GTK+ before 2.17.4 work properly
only when used with GTK+ before 2.17.4, and similarly when PSPPIRE is
compiled with GTK+ 2.17.4 or later.  There may be a way to avoid that, but
it is not entirely obvious.

With feedback from John Darrington.