By default, it equals to `15rem` (~ toolbar height and 300px view height with old behavior)

Other minor improvements:
 - `UFileMultiple`,`UFile`: not render an empty toolbar section if there are no buttons in it,
 - `UFileCollection`:
    * always render border splitter is the `activeRowSidebar` slot is passed,
    * add overflow auto for the `activeRowSidebar`