Implement library header strip control

#### Purpose

Rework headers interaction in `StandardPanel` to generalize and simplify them. Problems and proposals can be found [here](https://github.com/Benglin/Dynamo/pull/235#issuecomment-66562408).

#### Modifications

Created new control `HeaderStrip`. All interactions with headers is moved to that control. XAML of headers also added to new control.

#### Unit tests

![image](https://cloud.githubusercontent.com/assets/8158551/5537894/2a3626c4-8ab2-11e4-9210-8502ed7e018f.png)

#### Additional references

[MAGN-5635](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5635) DUI: Implement library header strip control

#### Reviewers

@Benglin, please, take a look.