Interface / Populate Empty Hand during Mining

[rlerner]

When you have the "hand" cursor (e.g. not holding any item) and you "mine" a building/belt/etc, populate the part in the user's hand to make movement of buildings more rapid.

[ssilk]

Good idea. Can be extended to:

- A key to swap between this and the last (the item before this) item in hand. (enables swapping of two items).
- If you placed the last item in hand while you craft that item then - when finished - the item pops up in hand while player does nothing (seems obviously to wait).