Handle complete path blockage

[nczempin]

trafficstars

It is possible to place towers such that no path to the base is possible.

We need to decide how to handle such cases. Some possibilities:

• spawn an extra creep that goes straight for blocking towers
• give the existing creeps extra tower-destroying capabilities
• do not allow path blocking towers to be placed

[nczempin]

completely removing any collision detection works for now. We need to teach the player that she is in this "mode". For now I will just play a siren and display a message.