DUI: improve search member group name.

# Description
This is created for a [previous pull request](https://github.com/Benglin/Dynamo/pull/139).

Please refer to [this article](http://msdn.microsoft.com/en-us/library/bb613560(v=vs.110).aspx) on MSDN and look under the section Avoid Using **TextBlock** in FlowDocument, then we know using **Run** element is better than having multiple **TextBlock** in terms of performance (I know the performance is not an issue for this case, but I'd prefer to have the right implementation). We should merge **SearchMemberGroup.Prefix** and **SearchMemberGroup.Name** into a single property that returns **Run objects** for **TextBlock.Content**.

Also, the last few comments in the same pull request for Prefix and GroupName should be fixed along with this.

# NB
Sorry, I couldn't get what you mean in this sentence: We should merge **SearchMemberGroup.Prefix** and **SearchMemberGroup.Name** into a single property that returns Run objects for TextBlock.Content. Could you, please, clarify it once more, so that I could add changes in this PR?

# Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5178](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5178) DUI: Improve SearchMemberGroup Name Display