Moving Addons TreeView to LibraryView

#### Purpose

Addons TreeView is situated in `LibraryContainerView` but logically it should be a member of `LibraryView`. In this PR `AddonsTreeView` is moved to correct place.

#### Modifications

XAML connected with `AddonsTreeView` is moved to `LibraryView`. Some connected with it methods is also moved to `LibraryView`. Additionally removed no more used methods from `LibraryContainerView`.

#### Additional references

[MAGN-4731](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4731).

At this point `AddonsTreeView` has it's own `ScrollViewer`. There was created task to remove this `ScrollViewer` - [MAGN-4969](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4969).

Also I am thinking about moving Addons TreeView to a separate UserControl. Because we need it in two places: LibraryView and LibrarySearchView. Anyway, it will one of next tasks.

#### Reviewers

Guys, sorry here is a lot of XAML code and can't do nothing with it. I did not created something new. I have removed unnecessary XAML and modified some code for a compatibility with LibraryTreeView. @aosyatnik, @Benglin, please, take a look.
What is the intention of commenting this out here? Is the underlying data still ```CustomNodeSearchElement```? If not what is it? Perhaps we need more extensive documentation for ```OnEditClick``` (about what it is expecting)?