DUI: navigate from top result(styling)

This PR should be merged after [that one](https://github.com/Benglin/Dynamo/pull/146).

# Preface
Next focus styling schema should be developed:
1. Focus on search text box. Top result is light grey.
2. Down key was pressed. Focus on top result. Top result is light grey.
3. Down key was pressed. Focus on first class/member. Top result is dark grey.

# Solution
I decided to select first top result item everytime, when top result gets focus.

# Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5008](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5008) DUI: While navigation from Top Result it background should be changed