CPTableView: When columns or rows dragging ends, enqueue the dragged views so they can be available when you drag again the column if they haven't been unloaded before.
The 2nd time you drag the same column/row, reduces -dragViewFor... duration by 90% according to webkit profiling.
Merge pull request #1478 from cacaodev/CPTableView-dragging-perf

CPTableView: make dragged views reusable when dragging ends