CMS-4270 Content "Shopping cart" no longer working

Moved `SelectionItem` to a separate file.
Implement usage of the `Viewer` in the `SelectionItem`.
Added `TreeRoot` class, that will handle selection and switching between
the default and filter panels.
CMS-4270 Content "Shopping cart" no longer working

Updated the behavior.
Merged `filteredSelection` and `defaultSelection` into the
`currentSelection` in `TreeRoot`.
Merge pull request #1038 from enonic/CMS-4270

CMS-4270 Content "Shopping cart" no longer working