Hide classes in search results

#### Purpose

EB asked us to hide classes in search results. We no need them at the moment. But should be left an option to restore them quickly if it needed.
![image](https://cloud.githubusercontent.com/assets/8158551/4898352/ab6dad12-640f-11e4-995c-ca6302b47fb1.png)

#### Modifications

In XAML we hide classes. No matter what binding property is. In code I am using preprocessor `#if`, `#else` with not defined in project's properties variable `SEARCH_SHOW_CLASSES` to change navigation logic when it should meet with classes.

#### Additional references

[MAGN-5301](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5301).

#### Reviewers

@aosyatnik, @Benglin, please take a look.
This part can be also moved to #if