ItemIdentifier returns false for null stack ...

... instead of crashing.
Pass onBlockActivated to TileEntity
Fix AbstractInventory enumerating from 2nd elem
SlotBase.onClick won't pick up more than a stack

In case we have slots with stacks of items higher than 64, make sure the
player doesn't pick up more items.
Fix SlotGroup adding in wrong order and positions
Small improvement in InventorySpaceEnumerator
Check stack limit in AbstractInventory isValid
Add InventoryCombined
Add base container and GUI code