DUI: hide code copy icon(no text inside)

![image](https://cloud.githubusercontent.com/assets/8158404/4844922/7189ec3c-6040-11e4-9461-a80cd45501da.png)
This PR should be merged after [that one](https://github.com/Benglin/Dynamo/pull/156).

Unfortunately, I couldn't find any node, that doesn't have description, so I couldn't test it. But I assume, that it works properly.

Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5203](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5203) DUI: Remove "Code" and "Copy" icons when there is no text between them