Add ListSelector web app add dialog support

[rjackey]

Manual validation to do beyond the automated R2025b tests:

1. R2024b+ Web App path
   Build/run a small Web App using wt.ListSelector, press Add, and verify the internal
   wt.dialog.ListSelection appears and works. Check OK, Cancel, close, modal overlay, and that
   selected items update Value/ValueIndex.

2. Selection behavior in the new dialog path
   In R2024b or newer, manually test:

   • AllowDuplicates=false with existing selections preselected.
   • AllowDuplicates=true appends duplicate selections.
   • ItemsData set, including numel(ItemsData) < numel(Items).
   • Cancel/close leaves Value unchanged.

3. Old-release desktop fallback
   In R2021a through R2024a, press Add and confirm it still uses listdlg() and preserves prior
   desktop behavior. R2021a matters because the toolbox still advertises R2021a+ runtime support.

4. Control visibility
   In a normal app and App Designer-created app, verify:

   • default shows Add/Delete/Up/Down
   • Sortable=false shows Add/Delete only
   • AllowAddRemove=false shows Up/Down only
   • both false shows no built-in buttons
   • user-added buttons still render/work

5. Potential future gate lowering
   For R2022a-R2024a, manually test whether wt.dialog.ListSelection works in Web App deployments. If
   it does, the R2024b gate may be lowerable later. Current implementation intentionally does not
   claim that.