Asks the TreeDragDest to insert a row before the
path dest, deriving the contents of the row from selection_data. If dest is outside the tree
so that inserting before it is impossible, false will be returned. Also, false may be returned if the new
row is not created for some model-specific reason. Should robustly handle a dest no longer found in the model!
Determines whether a drop is possible before the given dest_path, at the same depth as
dest_path. i.e., can we drop the data in selection_data at that location. dest_path does not
have to exist; the return value will almost certainly be false if the parent of dest_path doesn't exist,
though.