creative mode: set active block to the one targeted

[Gandrac]

To further speed up the building process I suggest a feature similar to the Photoshop Eyedropper Tool. It let's you select an already placed block, in order to keep building with this kind of block, but without removing the already placed one. To make this as easy as possible, the maximum amount of the selected block would just appear in the currently active slot of your hotbar.