ListBox Cmd-A should select all rows in multiple-selection mode

On macOS, when a window contained a ListBox with focus and multiple selection enabled, pressing Cmd-A did not select all rows. The standard Select All command should select every row in the focused multiple-selection ListBox, while editable cell text fields should continue to handle text selection themselves.

Fixed in the desktop runtime by routing Edit > Select All to focused ListBox controls, handling Cmd-A/Ctrl-A directly on the ListBox, and adding explicit ListBox.SelectAll(), ListBox.DeselectAll(), and ListBox.ClearSelection() APIs.