Add an AllowColumnResize property to ListBox

Desktop and Web ListBoxes currently always let the user resize columns by dragging the divider on a column header, and there is no way to switch that off. Add a read/write AllowColumnResize Boolean property (default True) to both the desktop and Web ListBox controls so apps can keep their column layout fixed, for example in report-style grids. When AllowColumnResize is False, the resize affordance is removed (the header divider gripper on desktop, the resize handle on web) and runtime resize attempts are rejected.

Column resizing continues to require visible headers (HasHeaders = True), matching Xojo and native grid controls; ColumnWidths remains available for programmatic width control regardless of this property.