Block selection fixes

Fixes #20904 (closed). When a "block" is selected from a Ioss output, a whole block and not just a partition is now correctly selected. This required that we introduce a new type of selection, BLOCK_SELECTORS, since composite-index based selections are no longer valid for PDCs in distributed environments since composite ids are different across ranks. Currently, we only create this new selection for PDCs and continue to create block-id based selection for other composite datasets. This may change in the future. A test has been added for this use-case.

Fixes #20899 (closed). Bring back support for context menus to change block appearance properties. A new class pqBlockContextMenu handles this. A test has been added for this use-case.

Fixes #20908 (closed). Ensure multi-file Ioss doens't end up with changing block ids. This is tested by test for #20899 (closed), so a separate test wasn't added.