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.~~

Just merge all tabs from gridFeature plugin in one. Layers must have identical attributes.
should be a real method as all other...