LP#1777207: add duplicate row detection to prepend()

LP#1777207: add duplicate row detection to prepend()

This patch ensures that if a visible grid item that might be
added via prepend() would duplicate an existing row entry, a
full collect() is done instead. This patch also converts a
prepend() to a full collect() if sorting is in effect, so we
cannot know whether the most recently added item should be
displayed in the visible rows otherwise.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>