Do not set titles on modal message dialogs, they are redundant with the contents

The GNOME HIG says: "Alert windows have no titles, as the title would usually
unnecessarily duplicate the alert's primary text."

Also remove useless icons on windows other than the primary PiTiVi window,
and change the "unsaved changes" dialog to a "question" defaulting to "Cancel".
render:  Show rendering errors in a dialog

Abort rendering when an error is encountered in the pipeline,
show the error so that the user actually knows something went wrong.

Fixes bug #635874
ui/filelisterrordialog: Set the GTK box orientation to be vertical

This also updates the GtkBuilder file's syntax to Glade's latest tastes
render: Show a more human-readable message when an error occurs in the pipeline

Hide the gory details in a GTK Expander widget