feat(UFileCollection,UFileMultiple): instroduce the `height` prop

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