view -- a gtk:tree-view widget
column -- a gtk:tree-view-column object being dragged
prev -- a gtk:tree-view-column object on one side of column
next -- a gtk:tree-view-column object on the other side of column
result -- true, if column can be dropped in this spot

Callback function type for determining whether column can be dropped in a particular spot as determined by prev and next. In left to right locales, prev is on the left of the potential drop spot, and next is on the right. In right to left mode, this is reversed. This callback function should return true if the spot is a valid drop spot. Please note that returning true does not actually indicate that the column drop was made, but is meant only to indicate a possible drop spot to the user.