Restyling headers on StandardPanel

#### Purpose

New design of headers on `StandardPanel`.

##### Before

![004](https://cloud.githubusercontent.com/assets/8158551/5232073/01806b7a-774f-11e4-80b0-a42c92a3d396.PNG)

##### After

![005](https://cloud.githubusercontent.com/assets/8158551/5232084/360c5d68-774f-11e4-9b2e-23e80dc31e97.PNG)

#### Modifications

XAML. Headers are made as buttons. Because they have behavior of buttons.

Removed one converter as not used anymore.

#### Additional references

[MAGN-5556](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5556).

We have situations when one of secondary headers are not presented. For that cases secondary header should have the same styling as primary header. Task for it [MAGN-5557](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5557).

#### Reviewers

@ramramps or @Benglin, please, take a look.