Used shouldDrag property getter instead of the ivar for more flexibility

This is for cases where a developer wants to automatically disable swiping depending on the current state of the cell or table view. In my case, I wanted to disable swiping if the table is in edit mode. Another approach is to set the shouldDrag property (using the setter) manually for each cell, but that will need more lines of code (need  to set on when edit mode becomes YES, and when edit mode becomes NO).