Fixed: In CPTableView, the drop indicator for rows could appear when dragging a column.

Previously, after drag & dropping rows, dragging a column while moving down the cursor was revealing the drop indicator.

This commit clears the types of the dragging pasteboard before dragging a column. This will avoid conflicts with the type declared for rows.
Fixed: -moveColumn:to: now preserve selected columns
Fixed: Starting a column drag is now faster.
Fixed: When dragging a selected column, selection is drawn on the dragging view
Fixed: The header view of a dragging column did not stay on the left edge

This commit reuses the views of the dragged column for the in-flight
column and caches the dragging view.