Add portals

[mkawalec]

Everything is better with portals. Choose if you want the portals to be bi or single directional, then add them on click. Portals preserve kinetic energy between the input and output nodes.

Bonus:

• let's pretend portals take some energy from the object when it's travelling through one. Take some kinetic energy from the hero when he is travelling, if the energy after travel goes below zero, kill the hero ;)