Make it possible to move obstacles when app is running

[akapkotel]

Obstacles are created and positioned at the start, and you can not change their positions after that.

It would be nice if you can click and grab an obstacle and move it to another place on the screen.

Requires: recalculating whole walls and corners on the screen before ray-casting algorithm can continue.

Requires: check if moved obstacle does not intersect with another one.