Replaced the wormspawner editor icon

I created a new symbolic icon that is distinguishable from the waypoint. My previous `W` is a little unclear.

![image](https://cloud.githubusercontent.com/assets/756669/5237611/5502d3a8-7891-11e4-9950-379ba3934697.png)

It is inspired by the icon from http://d2kplus.com/maprenderer.php but using a unicode char ⨂ instead of a Dune II sprite.