Secondary header style the same as primary header style if no right secondary header

#### Purpose

For situations when right secondary header in StandardPanel are not presented style of left secondary headers hould be the same as style of primary secondary header.

#### Modifications

`ControlTemplate` is moved to style. Added new converter which decides which style to use for left secondary header. So after PR next view is possible.
![001](https://cloud.githubusercontent.com/assets/8158551/5332576/38d6ec90-7e64-11e4-9beb-8c55e625988f.PNG)

#### Unit tests

Added unit test for new converter.
![001](https://cloud.githubusercontent.com/assets/8158551/5332582/5cb58a36-7e64-11e4-82bb-33b4d17e9b15.PNG)

#### Additional references

[MAGN-5557](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5557).

This PR should merged after this https://github.com/Benglin/Dynamo/pull/229.

#### Reviewers

@Benglin, please, take a look.