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
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".