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.
Hi @vmoyseenko, we can't use a physical file on disk. This won't work. Please refer to ```MapLogicalToVisualLineIndices``` method in solution for an example of how to construct this from a ```URI```. Also, I remember you were using ```TextBlock``` to get its fonts before this change, can we use that approach? I'm thinking if a font changes for ```TextBlock```, then perhaps we don't have to change the codes here. But we might not be able to access the ```TextBlock``` here I guess?