Resource extraction, minor changes to controls

Resource blocks (rocks, containing ore and trees containing wood) now contain 10 "units" of resources which the workers will gradually extract from them (by "jumping" next to them); when the block is depleted it is destroyed and the worker will look for another block.

There's a crude counter for displaying the current amount of available resources at the top left.

Controls are slightly altered once again, now you can fly over blocks while pressing holding the beam button to immediately start picking up the block.