Clarify ListBox designer column headers and multi-column row data

Improve ListBox designer support and documentation so column headers and multi-column row data are clear. The current Inspector field InitialColumns is easy to mistake for initial cell data, but it actually stores newline-delimited column header names and generates SetHeaderText(...) calls. Users also need explicit guidance that column values beyond column 0 should be populated with AddRow(Array(Of String)) or SetCellText(...).