Add SingleTab option in GridFeatures

If the option "singleTab" is true:
- All selected layers will be grouped in one grid (one tabpanel)
- This grid will contains each columns (from each layers) once
- if a layer doesn't have data for a given columns (from another layer), the correponding cell will be null.

This particular option have been developped for geoportal (or role) with only near-similar layers and the need of compare them.
Add space and indentation :-)