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.
I think @aosyatnik has a point. Even if they don't execute because of the ```return``` statement, it makes the change intentions much more hidden (and make it more error-prone), causes compiler warnings. Whereas the following change will tell the reader that *this part of the code does nothing at all*.

```cs
#if SEARCH_SHOW_CLASSES
    // No class is found in this 'SearchCategory', return from here so that higher level
    // element gets to handle the navigational keys to move focus to the previous category.
    if (searchCategoryContent.Classes.Count == 0)
        return;

    // Otherwise, we move to first class button.
    var listItem = FindFirstChildListItem(searchCategoryElement, "SubCategoryListView");
    if (listItem != null)
        listItem.Focus();

    e.Handled = true;
    
#else

    // All classes are hidden. Do not select any of them.

#endif
```

Also, we do not need ```return``` statements at all, removing them makes the code clearer.