Highlight of search phrase in class name

#### Purpose

Highlight search phrase in class name. Proposed design:
![image](https://cloud.githubusercontent.com/assets/8158551/4815817/44073f56-5eda-11e4-8251-5c6e575563a3.png)

#### Modifications

Added new converter `SearchHighlightClassMarginConverter`. It was hard to converter of members because in this case class name can be broken on two lines. We need handle both lines.

#### Additional references

[MAGN-4915](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4915).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.
The implementation of this is more complex than it should really be. Generally I'd like to see this being simplified as following steps:

1. Break ```fullText``` around ```\n``` into ```string[] lines``` with ```String.Split``` method.
2. Use ```foreach``` to go through these ```lines``` objects, find a match.
3. Once found, get its offset for left and right margins.

```cs
int lineIndex = 0;
string[] lines = fullText.Split("\n");
foreach(var line in lines)
{
    if (line.IndexOf(searchText) == -1)
        continue; // No match found on this line.
        
    // Caculate left and right offsets for margins.
    leftOffset = ...;
    rightOffset = ...;
    
    break; // Found the match on line 'lineIndex'.
}

// Use 'lineIndex' to compute the top, bottom margins. Since our fonts are of 
// the same height on each line, we can determine the top, bottom margins by 
// just knowing the 'lineIndex'.
var topMargin = lineIndex * ...;
var bottomMargin = topMargin + ComputeTextHeight(...);
```

This way we have a solution that's simpler to understand, and works for any number of lines.