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.
We have a flag that determines if we're under testing, ```DynamoModel.IsTestMode```. You could use that to determine if this list needs to be populated or not (if that makes things faster). You can change that later, let me merge this in first. Thanks again!