timeline: Do not disable update inserting a clip at the end of the timeline

Avoiding deadlocks, and anyway adding end is not consuming much
resources at the gnl level
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