LibraryView scrollable.

Task [MAGN-4380](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4380) The library should be scrollable using middle mouse wheel.
# Description:
Right now dragging the scroll bar is the only way to scroll.
# Solution:
Add `ScrollViewer`.

# Reviewers
@Benglin, please take a look.