Allow beforeStart callback to make dragtable temporarily do nothing on mousedown

Move the beforeStart callback above doing anything to allow the client to return false from it

and avoid dragtable from starting to do anything (i.e. make it inoperable)
We use this to disable dragging when a user is in-place editing a text field in the header
of the draggable table.  Without this, if they try to select their text, the table column
will drag and show a cloned empty text field.