Show / Hide add-ons header

#### Purpose

Implement logic to show / hide add-ons header depending on AddonRootCategories collection element availability.

#### Modifications

Binding to `Visibility` of label is added.

#### Additional references

[MAGN-4644](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4644).

#### Reviewers

@Benglin, this one is really simple. Please, take a look.