REACH-123 Add view for String node

[REACH-123 Add view for String node](http://adsk-oss.myjetbrains.com/youtrack/issue/REACH-123)

To work correctly, this PR requires changes in [Dynamo](https://github.com/Benglin/Dynamo/pull/241)
String node has textarea inside, so it needs Backspace and Delete buttons are allowed