log slider
- only add the gui logger after window is shown otherwise slider wont be shown
- need to silence gui logger after window closed otherwise logsystem hangs
- gui.GetLogLevelInt() extra function needed to give correcty int typecast value to gui widget that sets initial loglevel to default
gui stop
- introduce gui.Stop()
- remember state with open
- stopping ethereum stack is not gui concern, moved to main
- stopping mining, gui and ethereum handled via interrupt callbacks
- ^C triggers exactly the same behaviour as quit via menu