Improved items selectability check, and stop scrolling when an item is not selectable

[ccodega]

Main changes:

• removed getItemCount (was not used at all)
• renamed isIndexSelectable to isSelectable for consistency
• check items selectable also during selection, not only at first selection
• stop drag to select when an item cannot be selected
• added a DragToSelectListener in order to monitor activation/deactivation of drag to select