Add extra slots for core items (sword, pickaxe, block)

[kmcnaught]

At the moment in creative mode, attacking will use a sword, mining will use a pickaxe, and noob pole will use a block. All these are selected automatically so it would be really useful if they didn't take up space in the visible hotbar.

One intermediate solution might be to swap them into slot 1, and then swap back whatever was there in the first place when you stop attacking/mining/noob poling. Currently they go into an empty slot if there is one, but otherwise the inventory is allowed to choose which slot to put them in.