DUI: Replace BrowserItem.Focusable property

# Preface
Once it was discussed [here](https://github.com/DynamoDS/Dynamo/pull/2182), that it would be great to remove `Focusable` property.
# Solution
Instead of `Focusable` I created appropriate converter.
# Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4353](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4353) DUI: Replace BrowserItem.Focusable property