Remove IsSelected and SelectedIndex properties

#### Purpose

1. Remove `BrowserItem.IsSelected` property.
2. Remove `SearchViewModel.SelectedIndex` property

#### Modifications

Except changes specified in Purpose were removed `SearchViewModel.SearchResults`, `SearchViewModel.visibleSearchResults` as not needed. Navigation logic will be rewritten in a week.

#### Additional references

[MAGN-5076](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5076).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.