* src/io-context-gtk.c (struct _IOContextGtk): Add
transient_parent attribute.
(cb_icg_window_destroyed): Clear transient_parent attribute.
(icg_show_gui): Call gtk_window_set_transient if transient_parent
attribute is set.

* src/main-application.c (main): Call icg_set_transient_for after
each workbook is opened - yet another hack to keep the splash
screen on top of the workbooks.