Multiple base support, control enhancements

We've added support for multiple bases; enemies will choose the nearest one as their target when they are spawned.

When you start picking up a block, your movement speed is immediately reduced to 0 and you can not move while the pickup is in progress. You can stop the pickup by tapping the beam control button again or wait for it to finish.

If you keep holding the beam button after dropping a block, the block won't be picked up again once it's grounded; you'll have to release the button and press it again.

Pickup times are now different for various object types: raw materials (rocks, trees) take 3 seconds, built objects (towers, turrets) take 1.5 seconds, blueprints take 1 second.

Bugfix: it was possible to drop multiple turrets on towers.

Bugfix: the touch button for cycling blueprints was acting up.