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

ethereal/assets/qml/wallet.qml

@@ -319,7 +319,7 @@ ApplicationWindow {
 
 					Slider {
 						id: logLevelSlider
-						value: eth.getLogLevel()
+						value: eth.getLogLevelInt()
 						anchors {
 							right: parent.right
 							top: parent.top