Make Search for Addons

#### Purpose

Make search for Addons TreeView.

#### Modifications

To `SearchViewModel.SearchAndUpdateResults` added implementation of showing / hiding items of `AddonsTreeView`.

#### Additional references

[MAGN-4955](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4955).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.