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

ethereum/main.go

@@ -46,4 +46,8 @@ func main() {
 	}
 
 	utils.StartEthereum(ethereum, UseSeed)
+
+	// this blocks the thread
+  ethereum.WaitForShutdown()
+  ethlog.Flush()
 }