Auto-repeat page up/down on click-and-hold

_This issue created on behalf of @RandomGHUser, from [fin-hypergrid Issue #760](https://github.com/fin-hypergrid/core/issues/760):_

> Request the scrollbar functionality be enhanced such that holding down a mouse button on the bar (not the thumb) will scroll the grid to that position incrementally. This is a standard scrollbar function which does not seem to be currently implemented in hypergrid.