Shared drive tree grid with reparenting drag and drop and tri-state folder checkboxes

Nested file storage browser on the ReUI DataGrid: a recursive folder tree with getSubRows and DataGridTableRowExpand depth indentation, file manager style tree drag and drop that reparents, dropping on a folder to go inside it and between rows to reorder, with an insertion line and a folder tint as the only feedback so no row moves for the length of a drag, the drop depth read off the pointer's horizontal travel, a closed folder reachable by a plain vertical drag and opened once it receives a row, and a carried folder taking its whole subtree, tri-state folder checkboxes that cascade to every descendant, rolled up folder size and item count, expand all and collapse all controls, a bulk selection bar, and an honest empty state.

@reui