Don't cache item indexes in VisualDataModel.

Don't cache item indexes in VisualDataModel.

Sacrifice constant time lookup of indexes for rarely used features for
reduced memory usage.

Applications which use neither Packages, nor the VisualDataGroup api
don't ever need to query the index of an item so caching and maintaining
those indexes in unnecessary. Where they are used this add a cost
scanning for the index of an item in the cache, and in some instances
a lookup cost in the compositor.

Change-Id: I7d3c21a27a8a3c068c5ae2e1e326ff573c44d712
Reviewed-by: Martin Jones <martin.jones@nokia.com>