CPTableView: make dragged views reusable when dragging ends

... so they can be available (if not unloaded before) when you drag again a column/row with the same kind of views.

The 2nd time you drag the same column/row, reduces -dragViewFor... duration by 90% according to webkit profiling.

These views are either fresh views unarchived, or unloaded-reusable views. They are not visible views so it should be safe to add them to the cache instead of trashing them.