Arrange member groups depending on their availability rework

#### Purpose

Rework PR https://github.com/Benglin/Dynamo/pull/36 according the comment https://github.com/Benglin/Dynamo/pull/36#issuecomment-54774092.

#### Modifications

1. TextBlocks are created in XAML.
2. TextBlock's `Visibility`, `Text`, `FontWeight` properties are bound in XAML.
3. Converters created for `Text` and `FontWeight` properties.
4. Switching between styles is done in converter.

#### Additional references

[Link](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4521) to YouTrack task.

#### Reviewers

@aosyatnik, please take a look. 
I don't quite get the purposes of the above three ```SearchElementGroup``` when their values can be hard-coded inside the XAML. See XAML comments below.


I do not think we need to bind the ```TextBlock.Text``` property to a data, can we not simply set the ```Text``` property like following. The ```TextBlock.Text``` property will never change.

```xml
<TextBlock x:Name="secondaryHeaderLeft"
           Tag="Query"
           Foreground="#DDD"
           PreviewMouseDown="OnHeaderMouseDown"
           Text="QUERY">
    <Others... />
</TextBlock>
```

The same is applicable to ```SecondaryHeaderRightGroup``` below.