Support toggling check state for all selected items in a tree view

• open can.ex2, apply
• open Multiblock Inspector, and select a bunch of rows.
• Now, if you click on the checkbox in any of the selected rows to toggle it, the selection gets cleared.

What we want instead, is following:

• leave selection state unchanged
• change check-state for all selected items to match the item that was clicked on.

The same behavior is expected of all checkable tree/table widgets including the variables, blocks, side sets, etc. selection widgets for readers like Exodus, CTH.

See discussion #18150 (closed)