CMS-4357 Implement User Manager TreeGrid.
Fixed icons. Added item subname. Added modifiedTime column.
CMS-4357 Implement User Manager TreeGrid.
Enable actions depending on selected items.
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.