"No matches found" shouldn't appear if some AddonRootCategories element is presented

#### Purpose

1. "No matches button" doesn't take into account visibility of `AddonRootCategories` elements.
2. "Add-ons" label should be added above addons in search results.

#### Modifications

Created property `SearchViewModel.SearchAddonsVisibility`. It checks visibility of root categories. The property is used in decision to show "No matches found" button.

"Add-ons" label is moved to `AddonsTreeView` control.

#### Additional references

[MAGN-5058](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5058).
[MAGN-5077](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5077).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.
I see, well that's okay then. I believe there are many phases during WPF layout pass, so it seems like you have run into a case where the bound properties are not ready yet. There's no need for a task, thanks!