Create custom node icon on startPage

Issue: http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3868
@Benglin Please help me to review it. Thanks
The previous ```case``` did not have ```dvm.ShowStartPage = false``` and still the start page was hidden. That was because ```DynamoViewModel.Open``` which ```ShowOpenDialogAndOpenResultCommand``` eventually called, sets the value to ```false```. The reason is that the command may fail because user cancels it, in that case we should not set ```ShowStartPage``` to ```false```. Move the line into ```ShowNewFunctionDialogAndMakeFunction``` instead when everything is successful.