Fixed Bug for qucsator hang when simulation is aborted

Hello, Friends!

This bug is for Qucs0.0.17. If you abort long simulations by pressing button "Abort simulation", qucsator still running and not terminates. Qucsator terminates only after kill command. CPU load at this time is 100%. If I run new simulation, old qucsator processes still running until I kill them by command kill. I am using openSUSE12.2.
![hang-qucsact](https://f.cloud.github.com/assets/4920080/1459069/ff81ebc0-43bd-11e3-95e7-112a9f85eb88.png)

I fixed this bug by adding slot AbortSim() in file simmessage.cpp. This slot kills qucsator (SimProcess), when Abort button is pressed.