Highlight of search phrase in member name

#### Purpose

Highlight search phrase in member name. Proposed design is on screenshot.
![highlight1](https://cloud.githubusercontent.com/assets/8158551/4780809/5d064be0-5c79-11e4-8df8-359d1581aac8.PNG)

#### Modifications

Added `Rectangle` near each member's `TextBlock`. Created converter `SearchHighlightMarginConverter` which returns correct `Margin` for just added `Rectangle`.

#### Additional references

[MAGN-4916](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4916).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.
I'm wondering why did we not fold these ```FormattedText``` usage into a private helper method? It can be as simple as:

```cs
width = ComputeTextWidth(fullText.Substring(0, index + searchText.Length);
```