Fix crash in VisualDataModel drag selection example.

Fix crash in VisualDataModel drag selection example.

The crash was an assert on an invalid pre-condition, and that's simply
been removed.  But it did highlight another issue when moving items
due to separate code paths for iterating forwards and backward in
a composited list.  Basically when iterating backwards because we're
looking for the first instance of an index it's necessary to overshoot
a little to an index prior and then iterate forwards, and as such
there's little difference between iterating forward or backwards.

Change-Id: I6252e3e0170dc2c72d0204137c69275c8ccc519b
Reviewed-by: Martin Jones <martin.jones@nokia.com>