Virtual List - add 'scroll to top' button

[jakobz]

Description

It would be handy to have an Scroll-to-top button in Tables (and, probably, in other VirtualList-based components developed in projects).

Describe the solution you'd like

We need design for it. Something like and semi-transparent arrow at the bottom-left of the scrollable area.

Technically, it can be made like this:

• add 'renderScrollToTop' prop in the VirtualList. It can omit bothering about position. Just render anything passed inside VirtualLists's absolute positioning container.
• add 'ScrollToTopButton' component(s) in skin(s).
• add 'showScrollToTop' prop to the DataTable. It can be enabled by default. If passed, it would render the ScrollToTopButton from the skin.

[AlekseyManetov]

We add indexToScroll property in useVirtualList. It probably don't need to make any renderScrollToTop inside VirtualList, because it can be implemented by each UUI user manually according the project specific design.