small api addition in GTK+

["Matthias Clasen" <matthias clasen gmail com>]

See http://bugzilla.gnome.org/attachment.cgi?id=115614&action=view
in http://bugzilla.gnome.org/show_bug.cgi?id=424207

This is a harmless api addition that completes the family of
gdk_threads_ timeout functions by adding
gdk_threads_add_timeout_seconds and
gdk_threads_add_timeout_seconds_full.  This doesn't really
have the potential to break anything, and having it available means we
might do a little better in terms
of wakeups, which is nice for power saving.

There is another one or two small api additions that may happen in the
next few days.

Just to let you know,

Matthias