GtkArg changes

[James Henstridge <james daa com au>]

Some of the object args were broken by Tim's new arg <-> property
compatibility code.  A few of the new text widget args use
GTK_TYPE_POINTER and GTK_TYPE_BOXED, which spit out errors and cause a
segfault.  The list of args I have found so far is:
  GtkTextTag::font_desc
  GtkTextTag::tabs
  GtkTextView::tabs

James.