[HxGrid] select/deselect all checkbox when MultiSelectionEnabled with InfiniteScroll navigation (virtualized)

[hakenr]

Continuation of #950.
Basic support for multi-selection in the virtualized grid was implemented.

The select/deselect all checkbox is hidden in these scenarios since we don't have a complete list of items to select or deselect (there's no clear concept of "visible" rows as there is with paging).

A potential idea is to introduce a standalone bindable parameter like bool SelectAllChecked or a callback OnSelectAllCheckboxChanged, allowing developers to implement their own logic.